This is new in meson 0.40. Makes sure we find and use the
mesonintrospect from the same location as our meson, and
not some other meson version that just happens to be in the
path. We might be using meson directly from a checkout, for
example.
https://bugzilla.gnome.org/show_bug.cgi?id=781110
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