Re-declare GMenuModel / GActionGroup if we don't have a recent enough gio.
Fix a number of compile warnings.
use the standard gnumake macros / linking for ENABLE_GIO & ENABLE_DBUS
Initialize GDBusInterfaceVTable cleanly.
Take the Solar Mutex on hud_activated.

Change-Id: Ie0c1d6e75c4be857f0db1adad36418dd1693f63e