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.