Fix errors reported by clang++-2.9.

* src/evtgbaalgos/tgba2evtgba.cc (process_link): Fix prototype
to match tgba_reachable_iterator::process_link.
* src/ltlvisit/tunabbrev.hh: Add using super::visit, so that the
other visit() method are in scope when we overload one.
* src/tgba/tgbareduc.hh, src/tgba/tgbareduc.cc (start, end,
process_link): Remove these empty methods.  The default
implementations are empty too, and process_link had the
wrong prototype.
* src/tgbaalgos/reductgba_sim.hh, src/tgbaalgos/reductgba_sim.cc
(start, end, process_link): Likewise.

src/evtgbaalgos/tgba2evtgba.cc

@@ -64,7 +64,9 @@ namespace spot
       }
 
       virtual void
-      process_link(int in, int out, const tgba_succ_iterator* si)
+      process_link(const state*, int in,
+		   const state*, int out,
+		   const tgba_succ_iterator* si)
       {
 	// We might need to format out before process_state is called.
 	name_map_::const_iterator i = name_.find(out);