Restore _POSIX2_C_VERSION definition (bug 438).

Restore _POSIX2_C_VERSION definition (bug 438).

My review of conformtest expectations for POSIX showed up that the
_POSIX2_C_VERSION macro, required by POSIX and XPG standards before
2001, was missing in unistd.h, having been removed on 2003-04-03
despite those standards still being supported.  This patch adds it
back.  As it's in the implementation namespace, there's no need for it
to be conditional, and other such macros aren't conditional in this
header either.

Tested for x86_64 and x86 (testsuite).  Note that this *does* change
the installed libraries, because it affects the sysconf support
(present all along) for _SC_2_C_VERSION.

	[BZ #438]
	* posix/unistd.h (_POSIX2_C_VERSION): New macro.
	* conform/Makefile (test-xfail-POSIX/unistd.h/conform): Remove
	variable.