New makefile, for Windows cross-builds with clang-cl.

This was very strange to write, because it's a bizarre combination of
the GNU-make-isms and rc commands of Makefile.mgw with the cl and link
commands of Makefile.vc (but also the latter thankfully doesn't need
those horrible response files).

I've added a big comment in mkfiles.pl about what the build
requirements for this makefile actually are, which _hopefully_ will be
usable by people other than me.