stdint: respect system's intmax_t if INTMAX_MAX

stdint: respect system's intmax_t if INTMAX_MAX

* lib/stdint.in.h (intmax_t, uintmax_t): If the system defines
INTMAX_MAX, assume its intmax_t is OK.  Similarly for and
uintmax_t.  This is for some Mac OS X builds, where intmax_t is
long but int64_t is long long, and where we will clash with the
system intmax_t if we override it.  See
<http://lists.gnu.org/archive/html/bug-gnulib/2011-07/msg00160.html>.
(INTMAX_C, UINTMAX_C): For consistency, respect the system's
INTMAX_C if INTMAX_MAX and INTMAX_C are both defined, and
similarly for UINTMAX_C.