Create libmysql.a using dlltool.exe on mingw64

~~I have no idea if this will work.~~ It works! See #469.
This is probably more confusing than useful. I would remove the second line, so it only looks in `vendor/`. If there's a second search path, it should probably be a platform-specific path for fat binary scenarios (but I'd need some help setting that up, I didn't quite figure it out from your example repo).