Add mingw declarations

[nathaniel-daniel]

I added a mingw declarations file to support that platform. It was copied from the cygwin definitions. I appear to be able to compile, load, and use this library from msys2 with this change. __MINGW32__ is defined for 64 bit targets as well as 32 bit. I only tested this on mingw64 through msys2, but I think this should work for the original mingw as well, though I'm not sure if people still use that.