* src/ltlast/visitor.hh (visitor, const_visitor): Add empty

virtual destructors.
* src/tgba/tgbabddfactory.hh (tgba_bdd_factory): Likewise.
* src/misc/hash.hh: Use the std namespace only with GCC 3.0,
not with all compiler versions with minor version 0.
* src/tgba/tgbatba.cc (tgba_tba_proxy_succ_iterator): Fix
friend declaration of ::spot::tgba_tba_proxy.

src/tgba/tgbabddfactory.hh

@@ -33,6 +33,7 @@ namespace spot
   class tgba_bdd_factory
   {
   public:
+    virtual ~tgba_bdd_factory() {}
     /// Get the core data for the new automata.
     virtual const tgba_bdd_core_data& get_core_data() const = 0;
   };