* configure.in: check whether localtime caches TZ. copied from

	* configure.in: check whether localtime caches TZ.  copied from
	Emacs 20.5.
	* acconfig.h: add LOCALTIME_CACHE.

	* These changes should make it unnecessary to call tzset from
	Scheme after modifying the TZ environment variable, even if the
	system date facilities cache the value.
	* stime.c (setzone, scm_localtime): added comments.
	(tzset): don't define a noop tzset macro if HAVE_TZSET not defined.
	(setzone): don't call tzset.
	(restorezone): call tzset only if HAVE_TZSET is defined.
	(scm_tzset): don't define if HAVE_TZSET not defined.  Change the
	doc string to indicate that this procedure isn't likely to do
	anything useful.
	(scm_localtime, scm_strftime, scm_mktime): call tzset if
	LOCALTIME_CACHE is defined.