Fix #1484: search gettext subdir for header

On some systems, such as NetBSD, the gettext header is tucked under the
gettext directory in the system include area.  Let's add a path suffix
to ensure we correctly discover the header on such systems.
Merge pull request #1487 from jszakmeister/search-gettext-subdir

Fix #1484: search gettext subdir for header
Remove win16 references
Remove NeXTSTEP references
Remove VMS references
Remove Atari MiNT references
REVIEW AND REBASE REVIEW AND REBASE

First run at OS/2

NOTE: Paul Slootman was removed from the top of os_unix.c but is still
credited somewhere in the docs dir...