zeitgeitst doesn't compile with clang

Building my gentoo system with clang, I discovered that zeitgeist doesn't compile due to stricter casting rules:

/var/tmp/portage/gnome-extra/zeitgeist-0.9.14-r1/work/zeitgeist-0.9.14/extensions/fts++/controller.cpp:131:19: error:
      cannot cast from type 'gboolean (ZeitgeistFTS::Controller::*)()' to pointer type 'GSourceFunc'
      (aka 'gboolean (*)(gpointer)')
      g_idle_add ((GSourceFunc) &Controller::ProcessTask, this);
                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.