build: Disable pygobject submodule if system version exists

build: Disable pygobject submodule if system version exists

If pygobject is available on the system, its version is new enough,
don't build the older pygobject and rely on that system one.

This fixes totem not being able to use libpeas' Python support.

** (totem:544972): WARNING **: 12:04:05.407: Error initializing Python Plugin Loader: PyGObject initialization failed
ImportError: could not import gobject (version mismatch, 3.40.1 is required, found 3.38.1)

Closes: #806
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1135>