doc: Adjust INSTALL.md overall instructions

Change "package repositories" to "packaging systems".  Add Debian as
an example.  Caution that packaging systme bugs should be filed with
packaging systems.

Point to docs of prereqs, and do not attempt to open-code ocaml
instructions.  Fixes #966.

Change Unix-like to "mostly POSIX".

Note that a C compiler is needed; be vague for now until someone tries
with one that is too ancient, because no maintaened system should be
problematic.

INSTALL.md

@@ -14,14 +14,18 @@ choosing.
 
 ## Building from source
 
-### Package repositories
+### Packaging systems
 
-Many package repositories, including source-based repositories like `pkgsrc`,
-make it easy to build from source by handling all the dependencies for you.
-Please refer to instructions provided by the repository.
+Many packaging systems, including source-based systems like `pkgsrc`
+and binary repositories like Debian GNU/Linux, make it easy to build
+from source by handling all the dependencies for you and encoding the
+unison build recpipe.  Please refer to instructions provided by the
+packaging system.
 
+Issues with packaging systems should be filed with those systems, and
+not in the unison issue tracker.
 
-### Unix-like OS (GNU/Linux, BSDs, macOS, illumos-based OS, Solaris) and Cygwin
+### mostly-POSIX systems (GNU/Linux, BSDs, macOS, illumos-based OS, Solaris) and Cygwin
 
 #### Build prerequisites
 
@@ -30,8 +34,9 @@ Please refer to instructions provided by the repository.
   https://ocaml.org/.  (Note that ocaml upstream says that 5.x is
   experimental.  The standard approach is the most recent 4.x ocaml
   release.)
+- A non-ancient C compiler
 - GNU make
-- Basic POSIX tools: sh, sed (optional, for manuals)
+- Basic POSIX tools: sh, sed (optional, for manuals), perhaps more
 
 ##### Optional, for the GUI only
 
@@ -53,7 +58,14 @@ Please refer to instructions provided by the repository.
 
 #### Building
 
-Building from source is as simple as executing:
+To build from source, first ensure that all prerequisites are
+installed.  See each prerequisite's documentation for instructions, or
+use a package manager.  (Note that bugs in prerequisite's docs and
+packaging systems are not unison bugs and should not be reported in the
+unison issue tracker.)
+
+Building from source is as simple as changing to the source directory
+and executing:
 ```
 ${MAKE}
 ```
@@ -80,28 +92,6 @@ man/unison.1            (optional, manual page)
 doc/unison-manual.*     (optional, user manual in different formats)
 ```
 
-#### Building all from scratch
-
-It is very easy to build both the OCaml compiler and Unison from source without
-using a package repository. Building OCaml has very few prerequisites. If your
-system has a supported C compiler installed then the following may work out of
-the box:
-
-```
-# Build OCaml compiler
-## In a dir with extracted OCaml source
-./configure
-make
-make install    # you may need elevated privileges
-
-# Build unison
-## In a dir with extracted Unison source
-make
-```
-
-Building the GTK GUI this way is difficult as both GTK 3 and lablgtk3 must be
-built and installed.
-
 ### Cross-compiling for a different target architecture or OS
 
 To cross-compile for a different target, you need to have a cross-compilation