configure.ac: require GLib 2.12

Drop the required GLib version from 2.16 to 2.12, because many current
systems still don't have GLib 2.16.  This requires several new
compatibility functions in glib_compat.h.

src/state_file.c

@@ -22,6 +22,7 @@
 #include "playlist.h"
 #include "playlist_state.h"
 #include "volume.h"
+#include "glib_compat.h"
 
 #include <glib.h>
 #include <assert.h>