64-bit defines for int64_t and uint64_t

I hate patching third-party libraries because of the technical debt incurred for making patches to future versions of the library, but this file has already been patched by Mark Waddingham a year ago, and this does solve the problem.

Without this patch, there's a conflict between two definitions of int64_t and uint64_t when building on a 64-bit linux system. This solves the conflict and allows the compilation to continue.