tzset: Work around TZ problem on native Windows. * m4/tzset.m4 (gl_FUNC_TZSET): Require AC_CANONICAL_HOST. On native Windows, set REPLACE_TZSET to 1. * lib/tzset.c (tzset): On native Windows, fix TZ if necessary, and invoke '_tzset' instead of 'tzset'. * doc/posix-functions/tzset.texi: Mention the native Windows workaround. * modules/time_rz (Depends-on): Add tzset. * lib/time_rz.c (tzset): Remove fallback definition. * m4/time_rz.m4 (gl_TIME_RZ): Don't test for tzset.