The basic issue is that on BSDs at least, "make" is not GNU make.
On Debian GNU/Linux, GNU make is available as gmake, and this seems
normal.  Thus, use "gmake" as the example invocation of gmake,
consistently.  Leave the mac text as "make", as this is the only known
platform where "gmake" won't work.

When editing earlier, I missed the C99 text (in the ocaml bullet).
Hoist that to its own bullet.  Drop the redundant non-ancient C.

Move sed to doc prerequisites.  Add install and rm as basic POSIX
tools - but maybe we should remove the entire bullet point.

Be clearer about PREFIX and DESTDIR.

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.