Don't invoke localtime_r or gmtime_r unless it's the GNU C

	library's localtime_r and gmtime_r; there are too many buggy
	implementations of localtime_r and gmtime_r out there, and
	it's not worth keeping track of all the different bugs.

	* strftime.c (__EXTENSIONS__): Remove.
	(my_strftime_gmtime_r): Renamed from gmtime_r; all uses changed.
	(my_strftime_localtime_r): Renamed from localtime_r; all uses changed.
	Base them on localtime/gmtime if not _LIBC.