Don't assume that top-level checkout directory is openwebrtc

When included as a submodule in the openwebrtc repo, the top-level
directory might not always be cloned to have the same name as the repo.
girepository: Fix build

Remove the Python checks, since we drop everything that's built against
Python, and add gio-unix-2.0 deps for the build to complete
successfully.

Also girepository.h is needed for the seed build.
glib: Explicitly disable dtrace support

This is currently automagically detected, and breaks the static build
(glib_probes.o is generated outside of libtool, and libtool doesn't like
that).