* 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/misc/hash.hh

@@ -40,7 +40,7 @@
 #  else
 #    include <ext/hash_map>
 #    include <ext/hash_set>
-#    if __GNUC_MINOR__ == 0
+#    if __GNUC__ == 3 && __GNUC_MINOR__ == 0
       namespace Sgi = std;               // GCC 3.0
 #    else
       namespace Sgi = ::__gnu_cxx;       // GCC 3.1 and later