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).
libffi: Use autotools build system on iOS too

Fixes build with the iOS simulator and removes some more special casing.

https://github.com/EricssonResearch/openwebrtc/issues/70
gstreamer: Update to GIT master and build openh264 from there

https://github.com/EricssonResearch/openwebrtc-build-scripts/issues/27
seed: Update to latest GIT master for proper handling of GObject (transfer full) parameters
bootstrap: Update pygobject to 3.10.2

We were using 3.4.2 which is ages old. Updating to newer than 3.10.x would
require updating GLib and probably other things as well. We can look into that
when there is time.