Build doxygen pictures with libgd to reduce their size.

Doxygen only knows how to call dot with -Tpng, while using
-Tpng:gd produces pictures that are 10 times smaller.  Use a
simple wrapper around dot to simplify this.

* doc/dot.in: New file, that wrap the system's dot and replace
-Tpng by -Tpng:gd.
* doc/Makefile.am ($(srcdir)/stamp): Depend on dot.
* doc/Doxyfile.in: Update to 1.6.2.
(DOT_PATH): Set to @srcdir@ to use doc/dot instead of the
system's dot.
* configure.ac: Find the absolute path of dot, and generate
the doc/dot script.

doc/dot.in

@@ -0,0 +1,34 @@
+#!/bin/sh
+# Copyright (C) 2010 Laboratoire de Recherche et Développement de
+# l'Epita (LRDE).
+#
+# This file is part of Spot, a model checking library.
+#
+# Spot is free software; you can redistribute it and/or modify it
+# under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 2 of the License, or
+# (at your option) any later version.
+#
+# Spot is distributed in the hope that it will be useful, but WITHOUT
+# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+# or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public
+# License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with Spot; see the file COPYING.  If not, write to the Free
+# Software Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA
+# 02111-1307, USA.
+
+# Rewrite any -Tpng argument as -Tpng:gd, the resulting files are 10
+# times smaller because they are not anti-aliased.
+for arg
+do
+    case $arg in
+	-Tpng) set x "$@" -Tpng:gd;;
+	*)     set x "$@" "$arg";;
+    esac
+    shift # x
+    shift # $arg
+done
+
+@DOT@ "$@"