WebKitGTK depends on libwpe, but does not build with USE_LIBWPE. I guess this is correct, because we don't want to use as much of libwpe as WPE WebKit and PlayStation do, but it's really confusing and we should rethink it.
It's pretty hard to think of good alternatives for this, but I think we could rename it to USE(FULL_LIBWPE). Then it's much less confusing.