updated win mingw build via appyeyor CI

[chcg]

Replacement for #533

Probably mingw builds could also be added as cross platform build to the circle CI config. Similar to:

- docker run -v$PWD:/src -w /src ubuntu:bionic /bin/sh -c "apt-get -qq update && apt-get -qq install -y mingw-w64 && (for alt in i686-w64-mingw32-g++ i686-w64-mingw32-gcc x86_64-w64-mingw32-g++ x86_64-w64-mingw32-gcc; do update-alternatives --set \$alt /usr/bin/\$alt-posix; done); make -f Makefile.mingw BITS=$BITS"

[kevina]

@chcg are you willing to step up and be a maintainer for Windows builds?

I don't want to accept this without a Win32 maintainer as it will become out of date over time. I can fix the builds to some extent, but I won't be able to test any artifacts and, without a maintainer, I rather point people to MSYS2 builds.