Patch autogen.sh to make GTK-Doc really optional

• gnome-autogen.sh does’t recognize the need for GTK-Doc if GTK_DOC_CHECK
  is not present at the beginning of a line of configure.ac (which is
  the case here)

• autogen.sh now takes care of running gtkdocize, thus copying over
  gtk-doc.make. If GTK-Doc is not present, a fake gtk-doc.make is
  generated, so automake doesn’t complain anymore