dot: allow extra configuration via environment

* src/tgbaalgos/dotty.cc, src/taalgos/dotty.cc: Honnor the SPOT_DOTEXTRA
environement variable.
* src/tgbatest/readsave.test, wrap/python/tests/automata.ipynb: Test it.
* NEWS, src/bin/man/spot-x.x: Document it.

src/bin/man/spot-x.x

@@ -11,6 +11,12 @@ spot-x \- Common fine-tuning options.
 
 [ENVIRONMENT VARIABLES]
 
+.TP
+\fBSPOT_DOTEXTRA\fR
+The contents of this variable is added to any dot output, immediately
+before the first state is output.  This makes it easy to override
+global attributes of the graph.
+
 .TP
 \fBSPOT_SATLOG\fR
 If set to a filename, the SAT-based minimization routines will append

src/taalgos/dotty.cc

@@ -1,5 +1,5 @@
 // -*- coding: utf-8 -*-
-// Copyright (C) 2010, 2012, 2014 Laboratoire de Recherche et
+// Copyright (C) 2010, 2012, 2014, 2015 Laboratoire de Recherche et
 // Developpement de l Epita (LRDE).
 //
 // This file is part of Spot, a model checking library.
@@ -39,7 +39,11 @@ namespace spot
       void
       start()
       {
-        os_ << "digraph G {" << std::endl;
+        os_ << "digraph G {\n";
+
+	static const char* extra = getenv("SPOT_DOTEXTRA");
+	if (extra)
+	  os_ << "  " << extra << '\n';
 
         artificial_initial_state_ = t_automata_->get_artificial_initial_state();
 

src/tgbaalgos/dotty.cc

@@ -30,6 +30,7 @@
 #include "tgba/tgbagraph.hh"
 #include "tgba/formula2bdd.hh"
 #include "tgbaalgos/sccinfo.hh"
+#include <cstdlib>
 
 namespace spot
 {
@@ -108,6 +109,12 @@ namespace spot
 	  }
 	if (opt_circles_)
 	  os_ << "  node [shape=\"circle\"]\n";
+	// Any extra text passed in the SPOT_DOTEXTRA environment
+	// variable should be output at the end of the "header", so
+	// that our setup can be overridden.
+	static const char* extra = getenv("SPOT_DOTEXTRA");
+	if (extra)
+	  os_ << "  " << extra << '\n';
 	os_ << "  I [label=\"\", style=invis, ";
 	os_ << (opt_horizontal_ ? "width" : "height");
 	os_ << "=0]\n  I -> " << aut_->get_init_state_number() << '\n';

src/tgbatest/readsave.test

@@ -306,11 +306,14 @@ State: 3 "s3"
 [0] 1
 --END--
 EOF
-$autfilt -H input | $autfilt --dot=vcsn >output
+
+$autfilt -H input |
+    SPOT_DOTEXTRA='/* hello world */' $autfilt --dot=vcsn >output
 
 cat >expected <<EOF
 digraph G {
   node [shape="circle"]
+  /* hello world */
   I [label="", style=invis, height=0]
   I -> 3
   subgraph cluster_0 {