active -Wsuggest-override where supported

* m4/gccwarn.m4: Add the option.
* bin/autfilt.cc, bin/common_output.hh, bin/dstar2tgba.cc,
bin/ltl2tgba.cc, bin/ltl2tgta.cc, bin/ltlcross.cc, bin/ltldo.cc,
bin/ltlfilt.cc, bin/ltlgrind.cc, spot/kripke/kripke.hh,
spot/ltsmin/ltsmin.cc, spot/ta/ta.hh, spot/ta/tgtaproduct.hh,
spot/taalgos/dot.cc, spot/taalgos/reachiter.hh,
spot/taalgos/statessetbuilder.cc, spot/taalgos/stats.cc,
spot/twa/twaproduct.cc, spot/twaalgos/emptiness.cc,
spot/twaalgos/gtec/ce.cc, spot/twaalgos/lbtt.cc,
spot/twaalgos/ndfs_result.hxx, spot/twaalgos/stats.hh,
spot/twaalgos/tau03opt.cc, tests/core/ngraph.cc: Add suggested override
qualifiers.

spot/taalgos/dot.cc

@@ -141,7 +141,7 @@ namespace spot
       }
 
       void
-      start()
+      start() override
       {
         os_ << "digraph G {\n";
 
@@ -198,13 +198,13 @@ namespace spot
       }
 
       void
-      end()
+      end() override
       {
         os_ << '}' << std::endl;
       }
 
       void
-      process_state(const state* s, int n)
+      process_state(const state* s, int n) override
       {
 
         std::string style;
@@ -223,7 +223,7 @@ namespace spot
       }
 
       void
-      process_link(int in, int out, const ta_succ_iterator* si)
+      process_link(int in, int out, const ta_succ_iterator* si) override
       {
         bdd_dict_ptr d = t_automata_->get_dict();
         std::string label =

spot/taalgos/reachiter.hh

@@ -100,10 +100,8 @@ namespace spot
   public:
     ta_reachable_iterator_depth_first(const const_ta_ptr& a);
 
-    virtual void
-    add_state(const state* s);
-    virtual const state*
-    next_state();
+    virtual void add_state(const state* s) override;
+    virtual const state* next_state() override;
 
   protected:
     std::stack<const state*> todo; ///< A stack of states yet to explore.
@@ -118,10 +116,8 @@ namespace spot
   public:
     ta_reachable_iterator_breadth_first(const const_ta_ptr& a);
 
-    virtual void
-    add_state(const state* s);
-    virtual const state*
-    next_state();
+    virtual void add_state(const state* s) override;
+    virtual const state* next_state() override;
 
   protected:
     std::deque<const state*> todo; ///< A queue of states yet to explore.

spot/taalgos/statessetbuilder.cc

@@ -36,13 +36,13 @@ namespace spot
       }
 
       void
-      process_state(const state* s, int)
+      process_state(const state* s, int) override
       {
         states_set_.insert(s);
       }
 
       void
-      process_link(int, int, const ta_succ_iterator*)
+      process_link(int, int, const ta_succ_iterator*) override
       {
       }
 

spot/taalgos/stats.cc

@@ -38,7 +38,7 @@ namespace spot
       }
 
       void
-      process_state(const state* s, int)
+      process_state(const state* s, int) override
       {
         ++s_.states;
         if (t_automata_->is_accepting_state(s)
@@ -47,7 +47,7 @@ namespace spot
       }
 
       void
-      process_link(int, int, const ta_succ_iterator*)
+      process_link(int, int, const ta_succ_iterator*) override
       {
         ++s_.edges;
       }