Avoid running "dot" when it is not installed...

* src/tgbatest/kv.test: Do not run dot if it is not installed.
* iface/nips/nipstest/dotty.test: Likewise

iface/nips/nipstest/dotty.test

@@ -1,5 +1,5 @@
 #!/bin/sh
-# Copyright (C) 2008 Laboratoire de Recherche et Développement de
+# Copyright (C) 2008, 2011 Laboratoire de Recherche et Développement de
 # l'Epita (LRDE)
 #
 # This file is part of Spot, a model checking library.
@@ -26,6 +26,6 @@ set -e
 BYTECODE="never.pr.b regbit.b"
 for bytecode in $BYTECODE; do
     run 0 "$top_builddir/dottynips" "$srcdir/$bytecode" > dotty || exit 1
-    "$DOT" -Tps dotty > /dev/null || exit 1
+    "${DOT-:}" -Tps dotty > /dev/null || exit 1
     rm -f dotty
 done