mingw-w64: Add basic mingw-w64 support

Copied the compat stuff from libmicrodns.

a 6th grader could probably do better, this is some copying from stack overflow quality code. I hope someone can clean this up better than I can.

Builds and works as what I assume is intended on both ubuntu 20.04 gcc version 9.3.0 (Ubuntu 9.3.0-10ubuntu2) and windows using msys2's mingw-w64-x86_64-gcc cc version 10.1.0 (Rev3, Built by MSYS2 project) and mingw-w64 clang clang version 10.0.0 (https://github.com/msys2/MINGW-packages.git 7211ffb882cc3b7e7583c518aad45a22b278bc81)