INSTALL: rototill macOS section

Explain that macOS is a subset of mostly-POSIX, but that it's odd in
that normal tools and /usr/include are missing, and that the standard
approach is to install CLT or Xcode.

More or less, I think that if someone has a mac on which they can
build other POSIXY programs, unison will build fine.  If other mac GUI
programs build, unison mac GUI should build also.  And if the mac is
not set up to build programs, it's not really about unison, so a
CLT/Xcode hint suffices.

INSTALL.md

@@ -111,14 +111,21 @@ To build the documentation, first build Unison without cross-compilation.
 
 ### macOS
 
-#### Build prerequisites
+First, note that macOS is a mostly-POSIX system and see that section
+above.
 
-- Xcode Command Line Tools (optional, for the native GUI)
+Second, note that macOS by default is missing a number of expected
+tools, including POSIX-required system headers (e.g. all of
+/usr/include).  The standard approach on macOS is to install either
+Command Line Tools or Xcode, so that one has a working
+C99/mostly-POSIX environment.  These also allow building against the
+libraries required for the mac-native GUI.
 
 #### Building
 
-The Unix instructions above will build the text user interface, the GTK GUI
-and, if you're building on macOS, also the macOS native GUI.
+The mostly-POSIX instructions above will build the text user
+interface, the GTK GUI and, if you have native GUI headers/libs, also
+the macOS native GUI.
 
 To build only the macOS native GUI, execute:
 ```