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.