gobject-introspection: make g-ir-scanner try harder to find the right libgstreamer

gobject-introspection: make g-ir-scanner try harder to find the right libgstreamer

It's not quite clear to me why g-ir-scanner doesn't get this info from
the pkg-config file, nor why libtool doesn't get it from the .la.

https://bugzilla.gnome.org/show_bug.cgi?id=637549