gnome-base/gnome-panel uses cairo_xlib_surface_create, so it needs cairo[X] dep
Fixed in-tree, thanks for reporting.
Sorry for mess, I've just found that in gnome-panel-2.16.1 upstream changed code to use gdk_cairo_set_source_pixmap, which is gtk+ use of cairo's xlib surface. And gtk+ already has cairo[X], so I think that in gnome-panel-2.16.1 this usedep may be dropped (but not in 2.14*)
I was going to ask about this... Thanks for the update. I've backed out the X dep.