configury: set LIBRT only if it is required for clock_gettime.

* configure.ac (LIBRT): Extract from != Darwin guard and set to
-lrt if that is required for clock_gettime to work.
(POSIX_EXTRA_CFLAGS): Move up to compiler flag setting section.
(AC_CHECK_FUNCS): Move out of library checking section.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>