Test "ltl2tgba -FC" and plug the memory leaks of scc_map.

* src/tgbaalgos/scc.hh (scc_map::~scc_map): Declare it.
* src/tgbaalgos/scc.cc (scc_map::~scc_map): Implement it.
(scc_map::build_map): Delete duplicate states.
* src/tbbatest/ltl2tgba.test: Run ltl2tgba -FV to catch
memory leaks with valgrind.

src/tgbaalgos/scc.hh

@@ -64,6 +64,7 @@ namespace spot
     typedef std::set<bdd, bdd_less_than> cond_set;
 
     scc_map(const tgba* aut);
+    ~scc_map();
 
     void build_map();