The previous build files had a bunch of problems:

1. It assumed that glib would only be sourced via pkg-config
2. It was using the system gobject-introspection-1.0.pc file while
   building GIRepository-1.0.gir
3. It wasn't ignoring the *-autocleanup.h headers properly

Now you can build glib as a subproject and generate girs against the
in-tree sources. This also yields more accurate girs because they
document platform-specific features that are actually enabled in
the glib build we are linking against.