stdint: port to GCC 4.3 + OSX + Octave

stdint: port to GCC 4.3 + OSX + Octave

On this platform, stdint.h is buggy and defines int64_t to long
long int.  The replacement defined it to long int, causing
problems with C++ style name mangling.  Instead, trust the system
definition if INT64_MAX is defined, and likewise for the unsigned
variant.   Problem reported by Jarno Rajahalme in
<http://lists.gnu.org/archive/html/bug-gnulib/2010-04/msg00143.html>.
* lib/stdint.in.h (GL_INT64_T): Define if INT64_MAX is defined,
and don't mess with int64_t and INT64_MAX in this case.
(GL_UINT64_T): Likewise for UINT64_MAX and uint64_t.