The build_dir is an absolute path, so quote it.
We don't really need to use the absolute path to Ninja in git-update
since we pass it to subprocess.call which will do an shutil.which
This environment is meant to be setup in a Visual Studio
'Native X64 tools' terminal, and takes for granted that
msys2 has been installed, Visual Studio has C(++) support
has been installed and python3 is installed and is present
in PATH.
https://bugzilla.gnome.org/show_bug.cgi?id=775281