Add (void) casts for variables used only in assert()s.

Report from Akim Demaille.

* src/ltlvisit/snf.cc, src/ta/taexplicit.cc, src/taalgos/tgba2ta.cc,
src/tgbaalgos/dupexp.cc, src/tgbaalgos/stutter.cc: Here.

src/ltlvisit/snf.cc

@@ -56,6 +56,7 @@ namespace spot
 	visit(const constant* c)
 	{
 	  assert(c == constant::empty_word_instance());
+	  (void)c;
 	  result_ = constant::false_instance();
 	}