mktime: Work around TZ problem on native Windows. * lib/mktime.c: Add #ifs to make the algorithmic workaround independent from the native Windows workaround. * m4/mktime.m4 (gl_FUNC_MKTIME_WORKS): New macro, extracted from gl_FUNC_MKTIME. If guessing, set gl_cv_func_working_mktime to 'guessing no'. (gl_FUNC_MKTIME): Require it. Require AC_CANONICAL_HOST. Set REPLACE_MKTIME to 1 on native Windows. Define NEED_MKTIME_WORKING, NEED_MKTIME_WINDOWS. (gl_FUNC_MKTIME_INTERNAL): Require gl_FUNC_MKTIME_WORKS, not gl_FUNC_MKTIME. Set WANT_MKTIME_INTERNAL, not REPLACE_MKTIME. Define NEED_MKTIME_INTERNAL. * m4/timegm.m4 (gl_FUNC_TIMEGM): Require gl_FUNC_MKTIME_WORKS, not gl_FUNC_MKTIME. Cope with 'guessing yes' value. * modules/mktime-internal (configure.ac): Test WANT_MKTIME_INTERNAL, not REPLACE_MKTIME. * doc/posix-functions/mktime.texi: Mention the native Windows workaround.