Window support

This adds windows support. It's a bit flaky because the object files are compiled with mingw-w64 and for some reason require the function _chkstk_ms from libgcc. This is now included as an object file in the repo but in the future this should be changed.

Also, it is doing hacky things to cross-compile wolfMQTT. When https://github.com/wolfSSL/wolfMQTT/pull/178 is merged this flaky stuff can be removed.