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();
 	}
 

src/ta/taexplicit.cc

@@ -1,6 +1,6 @@
 // -*- coding: utf-8 -*-
-// Copyright (C) 2010, 2011, 2012, 2013, 2014 Laboratoire de Recherche
+// Copyright (C) 2010, 2011, 2012, 2013, 2014, 2015 Laboratoire de
-// et Développement de l'Epita (LRDE).
+// Recherche et Développement de l'Epita (LRDE).
 //
 // This file is part of Spot, a model checking library.
 //
@@ -363,6 +363,7 @@ namespace spot
       {
         state_ta_explicit* is = add_state(artificial_initial_state);
         assert(is == artificial_initial_state);
+	(void)is;
       }
   }
 

src/taalgos/tgba2ta.cc

@@ -61,6 +61,7 @@ namespace spot
 	  // unique artificial_livelock_acc_state
 	  assert(artificial_livelock_acc_state_added
 		 == artificial_livelock_acc_state);
+	  (void)artificial_livelock_acc_state_added;
 	  artificial_livelock_acc_state->set_livelock_accepting_state(true);
 	  artificial_livelock_acc_state->free_transitions();
 	}

src/tgbaalgos/dupexp.cc

@@ -56,6 +56,7 @@ namespace spot
 	unsigned ns = out_->new_state();
 	assert(ns == static_cast<unsigned>(n) - 1);
 	(void)ns;
+	(void)n;
       }
 
       virtual void

src/tgbaalgos/stutter.cc

@@ -398,6 +398,7 @@ namespace spot
 		  // No acceptance here to preserve the state-based property.
 		  i = a->new_transition(tmp, dst, one, 0U);
 		  assert(i > num_transitions);
+		  (void)i;
 		  all -= one;
 		}
 	    }