diff --git a/configure b/configure
index a740276ca..5c78fcbce 100755
--- a/configure
+++ b/configure
@@ -70,17 +70,23 @@ FLAG_datarootdir=''  # default initialized after processing flags
 FLAG_with_readline=''  # Fail if it's not available.
 FLAG_without_readline=''  # Don't even check if it's available
 FLAG_readline=''
-FLAG_with_systemtap_sdt=''  # Fail if it's not available.
 FLAG_without_systemtap_sdt=''  # Don't even check if it's available
+FLAG_without_libc_features=''
 
 # These variables are set by detect_readline and used by echo_cpp and
 # echo_shell_vars
 detected_deps=''
+
 have_readline=''
 readline_dir=''
 
 have_systemtap_sdt=''
 
+# libc
+have_fnm_extmatch=''
+have_glob_period=''
+have_pwent=''
+
 parse_flags() {
   while true; do
     case "$1" in
@@ -111,12 +117,12 @@ parse_flags() {
         FLAG_readline=$1
         ;;
 
-      --with-systemtap_sdt)
-        FLAG_with_systemtap_sdt=1
+      --without-systemtap-sdt)
+        FLAG_without_systemtap_sdt=1
         ;;
 
-      --without-systemtap_sdt)
-        FLAG_without_systemtap_sdt=1
+      --without-libc-features)
+        FLAG_without_libc_features=1
         ;;
 
       # TODO: Maybe prefix only needs to be part of the install step?  I'm not
@@ -269,25 +275,51 @@ detect_systemtap_sdt() {
   fi
 }
 
+detect_libc() {
+  if test -n "$FLAG_without_libc_features"; then
+    return
+  fi
+
+  # Check if non-POSIX FNM_EXTMATCH is available
+  if cc_quiet build/detect-fnm-extmatch.c; then
+    have_fnm_extmatch=1
+  fi
+
+  # Check if non-POSIX GLOB_PERIOD is available
+  if cc_quiet build/detect-glob-period.c; then
+    have_glob_period=1
+  fi
+
+  # Check if pwent is callable. E.g. bionic libc (Android) doesn't have it
+  if cc_quiet build/detect-pwent.c; then
+    have_pwent=1
+  fi
+}
+
 echo_shell_vars() {
   if test "$detected_deps" != 1; then
     die 'called echo_shell_vars before detecting readline.'
   fi
+
+  # Present a consistent interface to build/ninja-rules-cpp.sh
   if test "$have_readline" = 1; then
     echo 'HAVE_READLINE=1'
     echo "READLINE_DIR=$readline_dir"
   else
     echo 'HAVE_READLINE='
-    # Present a consistent interface to build/ninja-rules-cpp.sh
     echo 'READLINE_DIR='
   fi
+
+  # TODO: I don't think we need this in shell
   if test "$have_systemtap_sdt" = 1; then
     echo 'HAVE_SYSTEMTAP_SDT=1'
   else
     echo 'HAVE_SYSTEMTAP_SDT='
   fi
+
   echo "PREFIX=$FLAG_prefix"
   echo "DATAROOTDIR=$FLAG_datarootdir"
+
   if cc_quiet build/detect-cc.c -Wl,--gc-sections; then
     echo 'STRIP_FLAGS=--gc-sections'
   elif cc_quiet build/detect-cc.c -Wl,-dead_strip; then
@@ -433,21 +465,22 @@ echo_cpp() {
     echo '/* #undef HAVE_READLINE */'
   fi
 
+  # Used by mycpp/probes.h
   if test "$have_systemtap_sdt" = 1; then
     echo '#define HAVE_SYSTEMTAP_SDT 1'
   else
     echo '/* #undef HAVE_SYSTEMTAP_SDT */'
   fi
 
-  # Check if pwent is callable. E.g. bionic libc (Android) doesn't have it
-  if cc_quiet build/detect-pwent.c; then
-    echo '#define HAVE_PWENT 1'
+  if test "$have_fnm_extmatch" = 1; then
+    echo '/* libc defines FNM_EXTMATCH */'
   else
-    echo '/* #undef HAVE_PWENT */'
+    # INVALID value that we can detect in code
+    # e.g. musl libc does not support this, but glibc does
+    echo '#define FNM_EXTMATCH 0'
   fi
 
-  # Check if non-POSIX GLOB_PERIOD is available
-  if cc_quiet build/detect-glob-period.c; then
+  if test "$have_glob_period" = 1; then
     echo '/* libc defines GLOB_PERIOD */'
   else
     # INVALID value that we can detect in code
@@ -455,13 +488,11 @@ echo_cpp() {
     echo '#define GLOB_PERIOD 0'
   fi
 
-  # Check if non-POSIX GLOB_PERIOD is available
-  if cc_quiet build/detect-fnm-extmatch.c; then
-    echo '/* libc defines FNM_EXTMATCH */'
+  # Used by cpp/core.cc
+  if test "$have_pwent" = 1; then
+    echo '#define HAVE_PWENT 1'
   else
-    # INVALID value that we can detect in code
-    # e.g. musl libc does not support this, but glibc does
-    echo '#define FNM_EXTMATCH 0'
+    echo '/* #undef HAVE_PWENT */'
   fi
 }
 
@@ -480,9 +511,11 @@ main() {
   # Sets globals $have_readline and $readline_dir
   detect_readline
 
+  detect_libc
+
   detect_systemtap_sdt
 
-  # Generate configuration for oil-native
+  # Generate configuration for oils-for-unix
   local cpp_out=_build/detected-cpp-config.h
   echo_cpp > $cpp_out
   log "Wrote $cpp_out"
diff --git a/core/state.py b/core/state.py
index 69e6e9448..902bdda7d 100644
--- a/core/state.py
+++ b/core/state.py
@@ -36,6 +36,7 @@
                          NewDict)
 from pylib import os_path
 
+from libc import GLOB_PERIOD
 import posix_ as posix
 
 from typing import Tuple, List, Dict, Optional, Any, cast, TYPE_CHECKING
@@ -344,6 +345,14 @@ def _SetOptionNum(opt_name):
     return opt_num
 
 
+def _MaybeWarnDotglob():
+    # type: () -> None
+    if GLOB_PERIOD == 0:  # invalid value that means it wasn't detected
+        # GNU libc and musl libc have GLOB_PERIOD, but Android doesn't
+        print_stderr(
+            "osh warning: GLOB_PERIOD wasn't found in libc, so 'shopt -s dotglob' won't work")
+
+
 class MutableOpts(object):
 
     def __init__(self, mem, environ, opt0_array, opt_stacks, opt_hook):
@@ -379,6 +388,9 @@ def _InitOptionsFromEnv(self, shellopts):
 
     def Push(self, opt_num, b):
         # type: (int, bool) -> None
+        if opt_num == option_i.dotglob:
+            _MaybeWarnDotglob()
+
         overlay = self.opt_stacks[opt_num]
         if overlay is None or len(overlay) == 0:
             self.opt_stacks[opt_num] = [b]  # Allocate a new list
@@ -418,6 +430,8 @@ def _Set(self, opt_num, b):
 
         For bash compatibility in command sub.
         """
+        if opt_num == option_i.dotglob:
+            _MaybeWarnDotglob()
 
         # Like _Getter in core/optview.py
         overlay = self.opt_stacks[opt_num]
@@ -510,7 +524,7 @@ def _SetOldOption(self, opt_name, b):
             self.SetDeferredErrExit(b)
         else:
             if opt_num == option_i.verbose and b:
-                print_stderr('Warning: set -o verbose not implemented')
+                print_stderr('osh warning: set -o verbose not implemented')
             self._SetArrayByNum(opt_num, b)
 
         # note: may FAIL before we get here.
diff --git a/deps/from-tar.sh b/deps/from-tar.sh
index 4c235dcaa..0b54b7154 100755
--- a/deps/from-tar.sh
+++ b/deps/from-tar.sh
@@ -1,15 +1,16 @@
 #!/usr/bin/env bash
 #
-# Handle build dependencies that are in tarballs.
+# Handle build dependencies that are in tarballs, like the wild
 #
 # Usage:
 #   deps/from-tar.sh <function name>
 #
-# Examples:
-#   deps/from-tar.sh download-re2c
-#   deps/from-tar.sh build-re2c
+# For releases:
 #
-# The executable will be in ../oil_DEPS/re2c/re2c.
+#   deps/from-tar.sh configure-python
+#   deps/from-tar.sh build-python
+#
+# Note: this is not a tarball; it's in the repo.
 
 set -o nounset
 set -o pipefail
diff --git a/devtools/release.sh b/devtools/release.sh
index 984a7fd19..f4ca69193 100755
--- a/devtools/release.sh
+++ b/devtools/release.sh
@@ -6,7 +6,8 @@
 #   devtools/release.sh <function name>
 #
 # Steps:
-#   edit oils-version.txt, build/doc.sh update-src-versions, bump devtools/release-note.sh
+#   edit oils-version.txt, build/doc.sh update-src-versions, and
+#     bump devtools/release-note.sh
 #   $0 make-release-branch
 #   $0 two-tarballs          # CPython, then oils-for-unix, which is INSTALLED
 #   demo/osh-debug.sh osh-for-release: Start a shell to dogfood
diff --git a/doc/portability.md b/doc/portability.md
index d1f7d675c..b1e9609e5 100644
--- a/doc/portability.md
+++ b/doc/portability.md
@@ -30,8 +30,6 @@ which some libc's implement.
 
 This is unlike bash, which has its own glob library.
 
-TODO: `shopt -s dotglob` should give a warning when turned on.
-
 ### Atomic Assignments
 
 The signal handler assumes that int and pointer assignments are atomic.  This
diff --git a/frontend/py_readline.py b/frontend/py_readline.py
index 5c7eafad5..5f28b9075 100644
--- a/frontend/py_readline.py
+++ b/frontend/py_readline.py
@@ -5,6 +5,10 @@
 try:
     import line_input
 except ImportError:
+    # Note: build/ovm-compile.sh doesn't build pyext/line_input.c unless shell
+    # var $HAVE_READLINE is set
+    # On the other hand, cpp/frontend_pyreadline.cc uses -D HAVE_READLINE, a
+    # C++ preprocessor var
     line_input = None
 
 from typing import Optional, TYPE_CHECKING
diff --git a/osh/glob_.py b/osh/glob_.py
index 0afd5ac02..df939c37b 100644
--- a/osh/glob_.py
+++ b/osh/glob_.py
@@ -13,10 +13,11 @@
 )
 from core import pyutil
 from frontend import match
-from libc import GLOB_PERIOD
 from mycpp import mylib
 from mycpp.mylib import log, print_stderr
 
+from libc import GLOB_PERIOD
+
 from typing import List, Tuple, cast, TYPE_CHECKING
 if TYPE_CHECKING:
     from core import optview
diff --git a/test/configure-effects.sh b/test/configure-effects.sh
new file mode 100755
index 000000000..e0812933c
--- /dev/null
+++ b/test/configure-effects.sh
@@ -0,0 +1,62 @@
+#!/usr/bin/env bash
+#
+# What happens if various build features aren't detected?
+# In Python and C++
+#
+# Usage:
+#   test/configure-effects.sh <function name>
+
+set -o nounset
+set -o pipefail
+set -o errexit
+
+# bugs:
+# echo | tr
+# echo | cat
+# history | less
+
+prepare-cpp() {
+  ./NINJA-config.sh
+
+  # This overwrites the config
+  ./configure --without-readline --without-libc-features --without-systemtap-sdt
+
+  ninja
+}
+
+prepare-py() {
+  make
+}
+
+test-osh() {
+  local osh=$1
+
+  $osh -x -c 'set -o vi'
+  $osh -x -c 'set -o emacs'
+
+  # GLOB_PERIOD
+  $osh -x -c 'shopt -s dotglob'
+
+  # FNM_EXTMATCH
+  # Hm this will works
+  $osh -x -c 'echo */@(*.bash|*.asdl)'
+
+  # HAVE_PWENT
+  $osh -x -c 'compgen -A user'
+}
+
+cpp() {
+  #prepare
+
+  test-osh _bin/cxx-asan/osh
+}
+
+py() {
+  #prepare-py
+
+  ln -s -f -v oil.ovm _bin/osh
+
+  test-osh _bin/osh
+}
+
+"$@"