Force casts to please clang on OSX

* spot/twa/twagraph.hh, spot/twaalgos/minimize.cc,
spot/twaalgos/remfin.cc: Here.

spot/twa/twagraph.hh

@@ -434,7 +434,7 @@ namespace spot
 
     acc_cond::mark_t state_acc_sets(unsigned s) const
     {
-      assert(prop_state_acc() || num_sets() == 0);
+      assert((bool)prop_state_acc() || num_sets() == 0);
       for (auto& t: g_.out(s))
 	// Stop at the first edge, since the remaining should be
 	// labeled identically.
@@ -444,7 +444,7 @@ namespace spot
 
     bool state_is_accepting(unsigned s) const
     {
-      assert(prop_state_acc() || num_sets() == 0);
+      assert((bool)prop_state_acc() || num_sets() == 0);
       for (auto& t: g_.out(s))
 	// Stop at the first edge, since the remaining should be
 	// labeled identically.

spot/twaalgos/minimize.cc

@@ -660,7 +660,7 @@ namespace spot
     if (product(min_aut_f, aut_neg_f)->is_empty())
       {
 	// Complement the minimized WDBA.
-	assert(min_aut_f->prop_weak());
+	assert((bool)min_aut_f->prop_weak());
 	auto neg_min_aut_f = remove_fin(dtwa_complement(min_aut_f));
 	if (product(aut_f, neg_min_aut_f)->is_empty())
 	  // Finally, we are now sure that it was safe

spot/twaalgos/remfin.cc

@@ -153,7 +153,7 @@ namespace spot
 	     acc_cond::mark_t inf_alone,
 	     acc_cond::mark_t fin_alone)
     {
-      assert(aut->prop_state_acc());
+      assert((bool)aut->prop_state_acc());
 
       scc_info si(aut);
       // For state-based Rabin automata, we check each SCC for