class_gtkobject.m4: Put DOXYGEN_SHOULD_SKIP_THIS around *_Class prototypes.
class_gtkobject.m4: Put DOXYGEN_SHOULD_SKIP_THIS around *_Class prototypes.

This m4 file is used by gmmproc, which I have also corrected similarly.
These predeclarations, such as below, seem to confuse doxygen. See bug #743918 .

namespace Gtk
{ class Window_Class; } // namespace Gtk

GNOME/gtkmm,GNOME/gtkmm,johne53/MB3Gtkmm,johne53/MB3Gtkmm,GNOME/gtkmm,johne53/MB3Gtkmm,GNOME/gtkmm