This is required because in gcc-3.4 derived templates must reference
base template attributes explicitly.  Error example:

class_slot.h: In static member function `static SigC::SlotData* SigC::ClassSlot0_<R, Obj>::create(Obj*, typename SigC::ObjectSlot0_<R, Obj>::InFunc)':
class_slot.h:57: error: `callback' undeclared (first use this function)

diff -ur ardour-0.9beta16.1.old/libs/sigc++/sigc++/macros/class_slot.h.m4 ardour-0.9beta16.1/libs/sigc++/sigc++/macros/class_slot.h.m4
--- ardour-0.9beta16.1.old/libs/sigc++/sigc++/macros/class_slot.h.m4	2003-06-17 05:51:10.000000000 +0200
+++ ardour-0.9beta16.1/libs/sigc++/sigc++/macros/class_slot.h.m4	2004-06-14 22:45:39.906609600 +0200
@@ -77,7 +77,7 @@
       if (!obj) return 0;
       SlotData* tmp=manage(new SlotData());
       CallData &data=reinterpret_cast<CallData&>(tmp->data_);
-      data.callback=&callback;
+      data.callback=&[ObjectSlot]NUM($1)_<LIST(R,1,ARG_TYPE($1),[$1],Obj,1)>::callback;
       data.obj=&obj;
       data.func=(Func)func;
       return tmp;