Mention _POSIX_SOURCE feature test macro in man page.
PR: bin/19337
Submitted by: myself way back when I was a nobody :-) (schweikh)
Reviewed by: joerg, bde
MFC after: 2 weeks
Enforce that only Posix-spec'ed options are being used, so people could
use this as a Posixificator of some kind.
Document the Posix usage in the man page, so to make it clear what's
the difference between Posix c89, and gcc.
