Building gtk+-2.0.5-r2 fails (at least on all of my machines) unless I install gdk_pixbuf manually. I would think gdk_pixbuf should be in the dependency tree somewhere so it gets built automatically.
invalid, gdk_pixbuf is for gtk 1.2, gtk2 does not depend on it as it provides its own implementation of gdk_pixbuf (gdk) inside the gtk2 package now. This is some other error unspecified to my knowledge.