Cross platform builds with Clang

[DanRStevens]

Supposedly it's possible to do Windows builds from either Linux or MacOS using Clang. Perhaps we should figure out how that works.

I'm told the following two links might be useful:
https://github.com/llvm-mirror/llvm/blob/master/cmake/platforms/WinMsvc.cmake
https://github.com/mstorsjo/msvc-wine

The first one is a CMake file, for cross platform building, and the second is a Dockerfile for building with MSVC on Linux for CI setups.