Add cross-compilation Makefile for mingw32.

This is currently the only way to get asm optimizations in the decoder on
 Windows.
Builds both release and debug versions for each version of libc that currently
 ships with mingw32.
DLL version information must still be updated manually (at the top of the
 Makefile), but that's okay because mingw32 doesn't implement DllGetVersion
 anyway, so it will never show up unless someone queries the resources manually.

svn path=/trunk/theora/; revision=13601