The list of __HAVE macros

[Valery Ushakov <uwe%stderr.spb.ru@localhost>]

We don't seem to have(9) a man page that lists all __HAVE_* macros
that a port may provide.  E.g.

  $ apropos -M '"__HAVE_PREEMPTION"'
  cpu_need_resched (9)    context switch notification

but

  $ apropos -M '"__HAVE_SIMPLE_MUTEXES"'
  apropos: No relevant results obtained.
  Please make sure that you spelled all the terms correctly or try using different keywords.

I think it might be a good idea to document the list in one place with
xrefs to the relevant section 9 pages (and to document the options
there too, e.g. __HAVE_SIMPLE_MUTEXES in mutex(9)).

-uwe