Older versions have undefined behavior wrt. signed integer overflow.

https://bugs.gentoo.org/show_bug.cgi?id=267619
http://lists.gnu.org/archive/html/bug-gnulib/2006-12/msg00084.html

--- a/m4/mktime.m4
+++ b/m4/mktime.m4
@@ -145,7 +145,8 @@ int
 main ()
 {
   time_t t, delta;
-  int i, j;
+  int i;
+  unsigned int j;
 
   /* This test makes some buggy mktime implementations loop.
      Give up after 60 seconds; a mktime slower than that