Configure test for whether localtime caches TZ

[Ken Brown]

The configure test for whether localtime caches TZ directly modifies the 
environment and so (according to POSIX) might produce unreliable 
results.  Wouldn't it be better to use unsetenv, as in the following 
patch (against the emacs-23 branch):

=== modified file 'configure.in'
--- configure.in        2010-05-10 00:37:59 +0000
+++ configure.in        2010-10-30 18:07:03 +0000
@@ -2441,14 +2441,6 @@
 AC_CACHE_VAL(emacs_cv_localtime_cache,
 [if test x$ac_cv_func_tzset = xyes; then
 AC_TRY_RUN([#include <time.h>
-extern char **environ;
-unset_TZ ()
-{
-  char **from, **to;
-  for (to = from = environ; (*to = *from); from++)
-    if (! (to[0][0] == 'T' && to[0][1] == 'Z' && to[0][2] == '='))
-      to++;
-}
 char TZ_GMT0[] = "TZ=GMT0";
 char TZ_PST8[] = "TZ=PST8";
 main()
@@ -2458,13 +2450,13 @@
   if (putenv (TZ_GMT0) != 0)
     exit (1);
   hour_GMT0 = localtime (&now)->tm_hour;
-  unset_TZ ();
+  unsetenv("TZ");
   hour_unset = localtime (&now)->tm_hour;
   if (putenv (TZ_PST8) != 0)
     exit (1);
   if (localtime (&now)->tm_hour == hour_GMT0)
     exit (1);
-  unset_TZ ();
+  unsetenv("TZ");
   if (localtime (&now)->tm_hour != hour_unset)
     exit (1);
   exit (0);