Check more robustly for timer_settime.

Check more robustly for timer_settime.

This should fix an OS X build problem reported by Ivan Andrus in
<http://lists.gnu.org/archive/html/emacs-devel/2012-09/msg00671.html>.
* admin/merge-gnulib (GNULIB_MODULES): Add timer-time.
* configure.ac (gl_THREADLIB): Define to empty, since Emacs
does threads its own way.
* lib/gnulib.mk, m4/gnulib-comp.m4: Regenerate.
* m4/timer_time.m4: New file, from gnulib.
* src/atimer.c (alarm_timer, alarm_timer_ok, set_alarm, init_atimer):
Use HAVE_TIMER_SETTIME, not SIGEV_SIGNAL, to decide whether to
call timer_settime.