- Don't recommend libdvdnav, since DVD support isn't compiled by default
anymore.
- Take advantage of the new $MINGW_PACKAGE_PREFIX and $MSYSTEM_PREFIX
variables to make the build commands independent from the mingw-w64
build environment being used.
- Invoke /usr/bin/python3 directly, since I've heard some packages have
started to depend on mingw-w64 versions of Python, but our build
scripts only work with the MSYS2 version.
- Reword the MSYS2 install instructions to try to prevent common errors.