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

	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.

	* mktime.c (__EXTENSIONS__, HAVE_LOCALTIME_R): Remove.
	(my_mktime_localtime_r): Renamed from localtime_r; all uses changed.
	Base it on localtime unless _LIBC.