treat empty TZ environment variable as GMT rather than default

this improves compatibility with the behavior of other systems and
with some applications which set an empty TZ var to disable use of
local time by mktime, etc.

src/time/__tz.c

@@ -125,7 +125,8 @@ static void do_tzset()
 		"/usr/share/zoneinfo/\0/share/zoneinfo/\0/etc/zoneinfo/\0";
 
 	s = getenv("TZ");
-	if (!s || !*s) s = "/etc/localtime";
+	if (!s) s = "/etc/localtime";
+	if (!*s) s = __gmt;
 
 	if (old_tz && !strcmp(s, old_tz)) return;