mingw updates

Improve the state of the mingw build, updating dependencies to the latest working releases.

This sort of works, but only with the out-of-support openssl 1.0.2u release. That means we won't distribute a windows binary package from this tree, but it's a better reference for someone who wants to built it themselves.

The docker container is broken by the lack of ssl.dll which I couldn't get to build, but the following worked for me:

make -C mingw libopusfile-0.dll
make -C mingw package

And the results ran on a Windows 10 vm.