Allow building with Gtk+-3.0

Add a configure switch to select the Gtk+ version to build with. The
default is to build against Gtk+-2.0 >= 2.18.

https://bugzilla.gnome.org/show_bug.cgi?id=622303

src/Makefile.am

@@ -211,11 +211,11 @@ Meta-$(api_version).gir: $(G_IR_SCANNER) mutter $(libmutterinclude_HEADERS) $(mu
 		--namespace=Meta			\
 		--nsversion=$(api_version)		\
 		--include=GObject-2.0			\
-		--include=Gdk-2.0			\
-		--include=Gtk-2.0			\
+		--include=Gdk-@GTK_API_VERSION@		\
+		--include=Gtk-@GTK_API_VERSION@		\
 		--include=Clutter-1.0			\
 		--pkg=clutter-1.0			\
-		--pkg=gtk+-2.0				\
+		--pkg=gtk+-@GTK_API_VERSION@		\
 		--include=xlib-2.0			\
 		--include=xfixes-4.0			\
 		--program=$$pwd/mutter			\