PR modula2/110779 SysClock can not read the clock (Darwin portability fixes)

This patch adds corrections to defensively check against glibc functions,
structures and contains fallbacks.  These fixes were required under Darwin.

gcc/m2/ChangeLog:

	PR modula2/110779
	* gm2-libs-iso/SysClock.mod (EpochTime): New procedure.
	(GetClock): Call EpochTime if the C time functions are
	unavailable.
	* gm2-libs-iso/wrapclock.def (istimezone): New function
	definition.

libgm2/ChangeLog:

	PR modula2/110779
	* configure: Regenerate.
	* configure.ac: Provide special case test for Darwin cross
	configuration.
	(GLIBCXX_CONFIGURE): New statement.
	(GLIBCXX_CHECK_GETTIMEOFDAY): New statement.
	(GLIBCXX_ENABLE_LIBSTDCXX_TIME): New statement.
	* libm2iso/wrapclock.cc: New sys/time.h conditional include.
	(sys/syscall.h): Conditional include.
	(unistd.h): Conditional include.
	(GetTimeRealtime): Re-implement.
	(SetTimeRealtime): Re-implement.
	(timezone): Re-implement.
	(istimezone): New function.
	(daylight): Re-implement.
	(isdst): Re-implement.
	(tzname): Re-implement.

Signed-off-by: Gaius Mulley <gaiusmod2@gmail.com>