Allow getting system timezone information in libecal

Allow getting system timezone information in libecal

	** Part of fix for bug #381132

	* libecal/Makefile.am:
	* libecal/e-cal-system-timezone.h:
	* libecal/e-cal-system-timezone.c:
	New files for obtaining system timezone location,
	based on gnome-panel's clock-applet system-timezone.c.
	* libecal/e-cal-util.h: (e_cal_util_get_system_timezone_location),
	(e_cal_util_get_system_timezone):
	* libecal/e-cal-util.c: (): (e_cal_util_get_system_timezone_location),
	(e_cal_util_get_system_timezone):
	New API functions to obtain system timezone.