Sundown uses nmake on Windows

For building Sundown itself, they supply a makefile for use with MSVC's `nmake` that should be used.  `mix compile` still fails since `CC` and friends are still statically defined for GCC.  So, next I'll probably either move more initializations into the if/else or just create an `nmake` makefile.  The `clean` target still functions thanks to MSYS on my system.