OS X comes with a full X11 setup that it can run as a sort of `hosted' window system, distinct from the native system used by --with-ns. I would expect it to be fairly common to see developers on macosx with X11, Gnome, GTK, and perhaps even librsvg installed. Neither emacs not any other application of which I am aware *under macosx* can deal with both X11 and Carbon/Cocoa/Quartz at the same time, so simply finding the libraries installed is not sufficient to tell you (or configure) if the library should be used. It is possible that GNUStep systems can use both, since they implement one in terms of the other -- I'm unfamiliar with GNUStep these days.
*Chad