* m4/mktime.m4 (AC_FUNC_MKTIME): Sync from Autoconf.

* m4/mktime.m4 (AC_FUNC_MKTIME): Sync from Autoconf.
Include <limits.h>, and use its INT_MAX to rewrite the
j loop so that it does not overflow 'int'.  Problem reported by
Ralf Wildenhues in
<http://lists.gnu.org/archive/html/bug-gnulib/2006-12/msg00084.html>.
Play it safe by shifting left by 1 rather than multiplying by 2,
as GCC is less likely to optimize this away when the value
is signed (when it assumes overflow leads to undefined behavior).
Also, don't assume time_t uses two's complement.