Compile Using MinGW-w64 w/ pthreads

[jrbrtsn]

Recently I had a need to compile nats.c using MinGW-w64 w/ pthreads. Quickly I discovered an assumption in the project where pthreads is not to be used if _WIN32 is #define'd. After a few hours of hacking, I came up with a patch that makes possible the use of pthreads on Windows when using MinGW-w64 for the compiler. This resulting build has been tested, and works correctly. Since then I have submitted a pull request to the https://github.com/mxe/mxe project.

That said, the proper way to implement this support would involve rearranging the #define code inclusion logic.