How to cross-compile nordlicht for Windows

This is only for dynamic linking, haven't figured out how to do a static build yet.

1. Be on Arch Linux.
2. For convenience, add the mingw-w64 repository to pacman.
3. Install (at least) mingw-w64-ffmpeg, mingw-64-popt, and mingw-w64-mman-win32-svn.
4. Put this directory in the same directory as the nordlicht source code.
5. Pray to the gods of cross-compiling.
6. Run ./compile