Don't assume that meson is always a python script, on Windows it can
be (and soon will almost always be) an executable.
See: Meson MSI installer and https://github.com/mesonbuild/meson/pull/4004
This would not trigger on the build bot because there is
special casing in the code for a meson checkout underneath
gst-build. Fix needed as mesonintrospect.py was changed into
'meson.py introspect' in recent meson versions. When using
meson from git to configure the build the uninstalled script
would pick up a system meson instead which then would then
error out parsing the coredata from the newer meson.
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
