HAVE_UINT64_T not a good test

Hi again, last issue from me.

I noticed that you use HAVE_UINT64_T as a test to check if you should use uint64_t.

That does not work properly on GCC-64 (debian 4.4.5).  It will end up using the 32 bit code.

Perhaps a better test is checking UINT64_MAX, but I'm not sure.  I usually use CMake and all sorts of tests so I'm not sure what the shortest test could be.

cheers,
Paul
