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Might prevent builds with outdated tools.
Adds some additional MSBuild magic that chops dir names
from a given base, to avoid making deep, thin hierarchies
of dirs when building especially FFmpeg and libaegisub.
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).
Niels Martin Hansen