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).
Eliminates the need to have a copy of git accessible in the msys
build environment and reduces the dependency on msys to just FFmpeg.
Probably trivially faster due to how slow shell scripts are on Windows.
It seems that whitespace around the paths in the PATH, LIB, INCLUDE env
vars can break in some situations, making the build fail entirely.
Unfortunately this also makes the file harder to read and diff.
