Add first cut of the mingw cross build spec file + nsi file

These need more work but I'd rather have them here under version control.
The spec file appears to successfully build the Windows binaries, given
the right tar file to start with. Those binaries are then packed into an
rpm file (extermely useless to Windows users).

Once the rpm is unpacked one can then use the NSIS compiler and the .nsi
file to create a Windows installer. This all is still extremely fragile,
but it worked at least once...

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>