What does Brew do to Library path?

[jradxl]

Hello
I seem to get a problem if I try Brew on my Ubuntu 20.10 desktop...
After install, and when I run brew doctor, I get references to
Unexpected '.pc' files and Unexpected static libraries, which references to /usr/local/lib/
Most likely source stuff I've downloaded from the Internet and compiled myself, but I don't recognise any of the filenames.
(possibly before I installed Brew)

Also,
If I download a something.tar.gz from the Internet and try to run ./configure, I usually find that this package can't find dependency libraries that are correctly installed by apt-get and are already on my pc. If I look and the ./configure output, I can see the configure process was looking at a library path modified by Brew

I must be using Brew incorrectly.
Can someone explain...
Should I make a separate account on my pc, without Brew, just for the purpose of compiling Internet stuff?

[gromgit]

Can you share the output of brew doctor? For issues like this, the details often matter.

[jradxl]

Here it is...
I checked one of the files, and I had been compiling from source "xrdp" and "xorgxrdp", I think

$ brew doctor
Please note that these warnings are just used to help the Homebrew maintainers
with debugging if you file an issue. If everything you use Homebrew for is
working fine: please don't worry or file an issue; just ignore this. Thanks!

Warning: Unbrewed header files were found in /usr/local/include.
If you didn't put them there on purpose they could cause problems when
building Homebrew formulae, and may need to be deleted.

Unexpected header files:
  /usr/local/include/libavcodec/ac3_parser.h
  /usr/local/include/libavcodec/adts_parser.h
  /usr/local/include/libavcodec/avcodec.h
  /usr/local/include/libavcodec/avdct.h
  /usr/local/include/libavcodec/avfft.h
  /usr/local/include/libavcodec/d3d11va.h
  /usr/local/include/libavcodec/dirac.h
  /usr/local/include/libavcodec/dv_profile.h
  /usr/local/include/libavcodec/dxva2.h
  /usr/local/include/libavcodec/jni.h
  /usr/local/include/libavcodec/mediacodec.h
  /usr/local/include/libavcodec/qsv.h
  /usr/local/include/libavcodec/vaapi.h
  /usr/local/include/libavcodec/vdpau.h
  /usr/local/include/libavcodec/version.h
  /usr/local/include/libavcodec/videotoolbox.h
  /usr/local/include/libavcodec/vorbis_parser.h
  /usr/local/include/libavcodec/xvmc.h
  /usr/local/include/libavdevice/avdevice.h
  /usr/local/include/libavdevice/version.h
  /usr/local/include/libavfilter/avfilter.h
  /usr/local/include/libavfilter/buffersink.h
  /usr/local/include/libavfilter/buffersrc.h
  /usr/local/include/libavfilter/version.h
  /usr/local/include/libavformat/avformat.h
  /usr/local/include/libavformat/avio.h
  /usr/local/include/libavformat/version.h
  /usr/local/include/libavutil/adler32.h
  /usr/local/include/libavutil/aes.h
  /usr/local/include/libavutil/aes_ctr.h
  /usr/local/include/libavutil/attributes.h
  /usr/local/include/libavutil/audio_fifo.h
  /usr/local/include/libavutil/avassert.h
  /usr/local/include/libavutil/avconfig.h
  /usr/local/include/libavutil/avstring.h
  /usr/local/include/libavutil/avutil.h
  /usr/local/include/libavutil/base64.h
  /usr/local/include/libavutil/blowfish.h
  /usr/local/include/libavutil/bprint.h
  /usr/local/include/libavutil/bswap.h
  /usr/local/include/libavutil/buffer.h
  /usr/local/include/libavutil/camellia.h
  /usr/local/include/libavutil/cast5.h
  /usr/local/include/libavutil/channel_layout.h
  /usr/local/include/libavutil/common.h
  /usr/local/include/libavutil/cpu.h
  /usr/local/include/libavutil/crc.h
  /usr/local/include/libavutil/des.h
  /usr/local/include/libavutil/dict.h
  /usr/local/include/libavutil/display.h
  /usr/local/include/libavutil/downmix_info.h
  /usr/local/include/libavutil/encryption_info.h
  /usr/local/include/libavutil/error.h
  /usr/local/include/libavutil/eval.h
  /usr/local/include/libavutil/ffversion.h
  /usr/local/include/libavutil/fifo.h
  /usr/local/include/libavutil/file.h
  /usr/local/include/libavutil/frame.h
  /usr/local/include/libavutil/hash.h
  /usr/local/include/libavutil/hdr_dynamic_metadata.h
  /usr/local/include/libavutil/hmac.h
  /usr/local/include/libavutil/hwcontext.h
  /usr/local/include/libavutil/hwcontext_cuda.h
  /usr/local/include/libavutil/hwcontext_d3d11va.h
  /usr/local/include/libavutil/hwcontext_drm.h
  /usr/local/include/libavutil/hwcontext_dxva2.h
  /usr/local/include/libavutil/hwcontext_mediacodec.h
  /usr/local/include/libavutil/hwcontext_qsv.h
  /usr/local/include/libavutil/hwcontext_vaapi.h
  /usr/local/include/libavutil/hwcontext_vdpau.h
  /usr/local/include/libavutil/hwcontext_videotoolbox.h
  /usr/local/include/libavutil/imgutils.h
  /usr/local/include/libavutil/intfloat.h
  /usr/local/include/libavutil/intreadwrite.h
  /usr/local/include/libavutil/lfg.h
  /usr/local/include/libavutil/log.h
  /usr/local/include/libavutil/lzo.h
  /usr/local/include/libavutil/macros.h
  /usr/local/include/libavutil/mastering_display_metadata.h
  /usr/local/include/libavutil/mathematics.h
  /usr/local/include/libavutil/md5.h
  /usr/local/include/libavutil/mem.h
  /usr/local/include/libavutil/motion_vector.h
  /usr/local/include/libavutil/murmur3.h
  /usr/local/include/libavutil/opt.h
  /usr/local/include/libavutil/parseutils.h
  /usr/local/include/libavutil/pixdesc.h
  /usr/local/include/libavutil/pixelutils.h
  /usr/local/include/libavutil/pixfmt.h
  /usr/local/include/libavutil/random_seed.h
  /usr/local/include/libavutil/rational.h
  /usr/local/include/libavutil/rc4.h
  /usr/local/include/libavutil/replaygain.h
  /usr/local/include/libavutil/ripemd.h
  /usr/local/include/libavutil/samplefmt.h
  /usr/local/include/libavutil/sha.h
  /usr/local/include/libavutil/sha512.h
  /usr/local/include/libavutil/spherical.h
  /usr/local/include/libavutil/stereo3d.h
  /usr/local/include/libavutil/tea.h
  /usr/local/include/libavutil/threadmessage.h
  /usr/local/include/libavutil/time.h
  /usr/local/include/libavutil/timecode.h
  /usr/local/include/libavutil/timestamp.h
  /usr/local/include/libavutil/tree.h
  /usr/local/include/libavutil/twofish.h
  /usr/local/include/libavutil/tx.h
  /usr/local/include/libavutil/version.h
  /usr/local/include/libavutil/xtea.h
  /usr/local/include/libhandy-0.0/handy.h
  /usr/local/include/libhandy-0.0/hdy-action-row.h
  /usr/local/include/libhandy-0.0/hdy-animation.h
  /usr/local/include/libhandy-0.0/hdy-arrows.h
  /usr/local/include/libhandy-0.0/hdy-column.h
  /usr/local/include/libhandy-0.0/hdy-combo-row.h
  /usr/local/include/libhandy-0.0/hdy-deprecation-macros.h
  /usr/local/include/libhandy-0.0/hdy-dialer-button.h
  /usr/local/include/libhandy-0.0/hdy-dialer-cycle-button.h
  /usr/local/include/libhandy-0.0/hdy-dialer.h
  /usr/local/include/libhandy-0.0/hdy-dialog.h
  /usr/local/include/libhandy-0.0/hdy-enum-value-object.h
  /usr/local/include/libhandy-0.0/hdy-enums.h
  /usr/local/include/libhandy-0.0/hdy-expander-row.h
  /usr/local/include/libhandy-0.0/hdy-fold.h
  /usr/local/include/libhandy-0.0/hdy-header-bar.h
  /usr/local/include/libhandy-0.0/hdy-header-group.h
  /usr/local/include/libhandy-0.0/hdy-keypad.h
  /usr/local/include/libhandy-0.0/hdy-leaflet.h
  /usr/local/include/libhandy-0.0/hdy-list-box.h
  /usr/local/include/libhandy-0.0/hdy-main.h
  /usr/local/include/libhandy-0.0/hdy-paginator.h
  /usr/local/include/libhandy-0.0/hdy-preferences-group.h
  /usr/local/include/libhandy-0.0/hdy-preferences-page.h
  /usr/local/include/libhandy-0.0/hdy-preferences-row.h
  /usr/local/include/libhandy-0.0/hdy-preferences-window.h
  /usr/local/include/libhandy-0.0/hdy-search-bar.h
  /usr/local/include/libhandy-0.0/hdy-squeezer.h
  /usr/local/include/libhandy-0.0/hdy-string-utf8.h
  /usr/local/include/libhandy-0.0/hdy-swipe-group.h
  /usr/local/include/libhandy-0.0/hdy-swipeable.h
  /usr/local/include/libhandy-0.0/hdy-title-bar.h
  /usr/local/include/libhandy-0.0/hdy-value-object.h
  /usr/local/include/libhandy-0.0/hdy-version.h
  /usr/local/include/libhandy-0.0/hdy-view-switcher-bar.h
  /usr/local/include/libhandy-0.0/hdy-view-switcher.h
  /usr/local/include/libswresample/swresample.h
  /usr/local/include/libswresample/version.h
  /usr/local/include/libswscale/swscale.h
  /usr/local/include/libswscale/version.h
  /usr/local/include/lua/5.3/lapi.h
  /usr/local/include/lua/5.3/lauxlib.h
  /usr/local/include/lua/5.3/lcode.h
  /usr/local/include/lua/5.3/lctype.h
  /usr/local/include/lua/5.3/ldebug.h
  /usr/local/include/lua/5.3/ldo.h
  /usr/local/include/lua/5.3/lfunc.h
  /usr/local/include/lua/5.3/lgc.h
  /usr/local/include/lua/5.3/llex.h
  /usr/local/include/lua/5.3/llimits.h
  /usr/local/include/lua/5.3/lmem.h
  /usr/local/include/lua/5.3/lobject.h
  /usr/local/include/lua/5.3/lopcodes.h
  /usr/local/include/lua/5.3/lparser.h
  /usr/local/include/lua/5.3/lprefix.h
  /usr/local/include/lua/5.3/lstate.h
  /usr/local/include/lua/5.3/lstring.h
  /usr/local/include/lua/5.3/ltable.h
  /usr/local/include/lua/5.3/ltm.h
  /usr/local/include/lua/5.3/lua.h
  /usr/local/include/lua/5.3/luaconf.h
  /usr/local/include/lua/5.3/lualib.h
  /usr/local/include/lua/5.3/lundump.h
  /usr/local/include/lua/5.3/lvm.h
  /usr/local/include/lua/5.3/lzio.h
  /usr/local/include/painter.h
  /usr/local/include/rfxcodec_common.h
  /usr/local/include/rfxcodec_decode.h
  /usr/local/include/rfxcodec_encode.h
  /usr/local/include/xrdp_client_info.h
  /usr/local/include/xrdp_constants.h
  /usr/local/include/xrdp_rail.h
  /usr/local/include/xrdp_sockets.h

Warning: Unbrewed '.la' files were found in /usr/local/lib.
If you didn't put them there on purpose they could cause problems when
building Homebrew formulae, and may need to be deleted.

Unexpected '.la' files:
  /usr/local/lib/libpainter.la
  /usr/local/lib/librfxencode.la

Warning: Unbrewed '.pc' files were found in /usr/local/lib/pkgconfig.
If you didn't put them there on purpose they could cause problems when
building Homebrew formulae, and may need to be deleted.

Unexpected '.pc' files:
  /usr/local/lib/pkgconfig/libavcodec.pc
  /usr/local/lib/pkgconfig/libavdevice.pc
  /usr/local/lib/pkgconfig/libavfilter.pc
  /usr/local/lib/pkgconfig/libavformat.pc
  /usr/local/lib/pkgconfig/libavutil.pc
  /usr/local/lib/pkgconfig/libpainter.pc
  /usr/local/lib/pkgconfig/libswresample.pc
  /usr/local/lib/pkgconfig/libswscale.pc
  /usr/local/lib/pkgconfig/rfxcodec.pc
  /usr/local/lib/pkgconfig/xrdp.pc

Warning: Unbrewed static libraries were found in /usr/local/lib.
If you didn't put them there on purpose they could cause problems when
building Homebrew formulae, and may need to be deleted.

Unexpected static libraries:
  /usr/local/lib/libavcodec.a
  /usr/local/lib/libavdevice.a
  /usr/local/lib/libavfilter.a
  /usr/local/lib/libavformat.a
  /usr/local/lib/libavutil.a
  /usr/local/lib/libpainter.a
  /usr/local/lib/librfxencode.a
  /usr/local/lib/libswresample.a
  /usr/local/lib/libswscale.a

[gromgit]

OK thanks. To address your questions:

After install, and when I run brew doctor, I get references to
Unexpected '.pc' files and Unexpected static libraries, which references to /usr/local/lib/
Most likely source stuff I've downloaded from the Internet and compiled myself, but I don't recognise any of the filenames.
(possibly before I installed Brew)

brew doctor is warning you about these because /usr/local is a common hierarchy that most open-source software will install to by default...and look at to find the stuff needed during compilation. Homebrew's merely warning you that if you're forced to build from source for whatever reason, the software's own build process may pick up those files instead of the carefully-curated and -tested ones inside Homebrew. If your Homebrew installs are always from pre-compiled bottles, then this is a non-issue.

If I download a something.tar.gz from the Internet and try to run ./configure, I usually find that this package can't find dependency libraries that are correctly installed by apt-get and are already on my pc. If I look and the ./configure output, I can see the configure process was looking at a library path modified by Brew

More details are needed, in particular: What is this "library path modified by Brew"? An environment variable?

[jradxl]

Thanks for the clarification of my first question.

The user account I've been using is very old, probably since 19.x and upgraded to currently 20.10, with many things loaded and unloaded (via .bashrc). And I've noticed this rather odd experience of things won't compile because they can't find the right libraries. (I've been commenting out Brew lines in .bashrc temporarily to get a compile).

I thought I'd try the other way round and I created a brand new user account with only Brew additionally loaded, and behold the compile problem goes away.
I tried "msmtp-1.8.15.tar.xz" and in the .configure output I can see...

checking for ld used by gcc... /home/linuxbrew/.linuxbrew/bin/ld
checking if the linker (/home/linuxbrew/.linuxbrew/bin/ld) is GNU ld... yes

I'd also removed "/home/linuxbrew/" as well and reinstalled.
So it looks like it has been either a previously damaged install in "/home/linuxbrew/" or something else loading that were interacting.
Clearly it's not Brew and coincidence that temporarily removing Brew resolved it.
Sorry for the inconvenience and I'll research more to see if I can resolve this issue myself.