Log Message:
harfbuzz: Ensure glib2 is included before gtk-doc.

This works around an issue with detecting whether all required libraries are
runtime dependencies.  gtk-doc is a build-only dependency, which causes bl3
to tag glib2 and all of its dependencies as build too.  Later inclusion of
glib2 marks it as a full dependency, but include guards mean all of its
dependencies are not.  Including glib2 first is a hack, but does resolve the
problem for now.