Allow cross-compiling with mingw64.

With these changes I was able to cross compile for windows from a linux box. (Using the mingw-w64 package on Debian Testing).

Fixed a bug where the `target_family` cfg would be wrong when targeting something with a different value than the host. (i.e windows -> unix or unix -> windows). Also, a small issue with `LIBUV_FLAGS` not being set properly for the targets.

After this we can create a snapshot and migrate to mingw64 instead of mingw32.