patch 8.1.1567: localtime_r() does not respond to $TZ changes

Problem:    Localtime_r() does not respond to $TZ changes.
Solution:   If $TZ changes then call tzset(). (Tom Ryder)
diff --git a/src/config.h.in b/src/config.h.in
index c1ced6f..f9ae0bb 100644
--- a/src/config.h.in
+++ b/src/config.h.in
@@ -217,6 +217,7 @@
 #undef HAVE_TOWLOWER
 #undef HAVE_TOWUPPER
 #undef HAVE_ISWUPPER
+#undef HAVE_TZSET
 #undef HAVE_UNSETENV
 #undef HAVE_USLEEP
 #undef HAVE_UTIME