CONFIGURE/UNITY: Check for Glib version and suppress warnings in unity-taskbar.cpp

Here is the deal: g_type_init() is deprecated and does nothing except raising a number of ugly warnings in GLib >= 2.36, and UnityTaskbarManager::UnityTaskbarManager() does in fact call g_type_init().

This irks me, so I tried to fix this. 

So, if Unity is enabled:

1. We check for glib version
2. If < 2.36 we set a flag in config.h
3. We #ifdef against that in unity-taskbar.cpp

Note that this is completely unfamiliar territory for me (both configure and unity), so this might be very wrong or suboptimal or... well. 

You tell me.