This deliberately doesn't perform any font substitutions as the sort of
people that use libass on Windows tend to perfer to manually pick
fonts with the correctly glyphs.
Has no real effect, it's what VS 2013 already uses and detailed
logging reveals that the build system doesn't even recognize v4
and assumes v12 anyway.
Might prevent builds with outdated tools.
This eliminates the need for a msys environment to build any part of
Aegisub. Also happens to make building FFmpeg much faster (~3 minutes
vs. ~10 minutes for me).
