Support building with MinGW-w64 toolchain

[PatrickM-ZS]

This should fix #2051. Thanks to @e-tmplr!

I've added a Dockerfile for Linux with a cross-compile toolchain. It builds, but I've not had much success running the binaries through wine. 32-bit crashes with illegal memory access, 64-bit runs but complains about malformed packets and outputs some gibberish. Maybe due to wine bugs, bad configuration, or perhaps there's something more serious. But that could be fixed on a different PR.