now use unsigned __int64 for time type when on MSVC, that is supported

now use unsigned __int64 for time type when on MSVC, that is supported
down to at least MSVC6 (thanks Sisyphus!)
Also, type present checks are not done if stdint or inttypes headers are
found, no need for them then.

git-svn-id: svn+ssh://svn.code.sf.net/p/freeglut/code/trunk/freeglut/freeglut@1116 7f0cb862-5218-0410-a997-914c9d46530a