Well, you could install the gtk-devel package for SuSe. I don't know if they make the sources available or not.
You could try building GTK+ from the official, stock sources found at the
GTK+ website, but I can't predict how they will work with the rest of your system.
I would get the gtk-devel package(s) for SuSe (however that is done) and then proceed with your Mplayer build.
Peace...