Compile correctly under mingw32, mingw64

Hello!  We're likely to start using scrypt for key encryption in Tor, so I'm evaluating scrypt implementations.

We need our scrypt implementation to build and run on Windows, at least with the mingw version of GCC.  Fortunately, it wasn't too hard to get it going.  Here are the code changes I needed.  Please let me know if any of this isn't self-explanatory.