Since gtk-doc is an optional dependency for Cogl then we need to special
case how EXTRA_DIST is initialized. It shouldn't be set when using
gtk-doc since gtk-doc.make expects to initialize EXTRA_DIST. If we
aren't using gtk-doc then it should be initialized to an empty value
instead of including gtk-doc.make so that the later lines that append
various extra png files to EXTRA_DIST won't fail.
0e5df17761
