Replace non-standard __int64 with portable int64_t

The type __int64 is recognized by GCC and Visual C++ but may not work on
other compilers. In particular, Clang (the default compiler on Mac OS X)
does not recognize __int64. To resolve this issue, I'm adding the
<stdint.h> header file and replaced the non-standard type with int64_t,
which is standardized and will work on any compiler which supports
64-bit integers.