[gtkmm] class_gtkobject.m4: Put DOXYGEN_SHOULD_SKIP_THIS around *_Class prototypes.

[Murray Cumming <murrayc src gnome org>]

commit 00fa70e71b2637735360cce56f9f1bc2afc8674a
Author: Murray Cumming <murrayc murrayc com>
Date:   Tue Feb 3 12:51:20 2015 +0100

    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

 tools/m4/class_gtkobject.m4 |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)
---
diff --git a/tools/m4/class_gtkobject.m4 b/tools/m4/class_gtkobject.m4
index 6c721bf..563e8cb 100644
--- a/tools/m4/class_gtkobject.m4
+++ b/tools/m4/class_gtkobject.m4
@@ -81,7 +81,10 @@ define(`_END_CLASS_GTKOBJECT',`
 _SECTION(SECTION_HEADER1)
 _STRUCT_PROTOTYPE()
 
+#ifndef DOXYGEN_SHOULD_SKIP_THIS
 __NAMESPACE_BEGIN__ class __CPPNAME__`'_Class; __NAMESPACE_END__
+#endif //DOXYGEN_SHOULD_SKIP_THIS
+
 _SECTION(SECTION_HEADER3)
 
 namespace Glib