The unreleased G++-3.4 is more strict about not accepting extra
qualifications.  That is to say, you can't put on a Scope:: to the front of
a declaration if you're already in that scope.

enums.hg has this code:

namespace Glib
{
 
template <>
class Glib::Value<Gtk::IconSize> : public Glib::Value_Enum<Gtk::IconSize>
{
public:
  static GType value_type() G_GNUC_CONST;
};
                                                                          
                          
} // namespace Glib

You need to get rid of the first Glib:: making the line:

class Value<Gtk::IconSize> : public Glib::Value_Enum<Gtk::IconSize>

The other declarations in enums.h seem to come from elsewhere and don't
have this problem.