sdk/glib: Build introspection information from glib itself

Till now introspection information for glib was provided by gobject-introspection, but now it's merged into glib itself.

This means that there's a small cyclical dep we need to break, and thus we have to build glib without introspection, then build gobject-introspection for the libraries, and then build our glib with introspection.

Additionally, we rebuild gobject-introspection against our final glib, with build_introspection_data=true to have a clean separation of builds. Note that this isn't technically required and downstreams could build gobject-introspection once, and then change the runtime glib dependencies to the final one.