ltlcross: extract the color handling code

* bin/common_color.cc, bin/common_color.hh: New files, with
code extracted from ltlcross.cc.
* bin/Makefile.am: Add them.
* bin/ltlcross.cc: Simplify.

bin/common_color.hh

@@ -0,0 +1,31 @@
+// -*- coding: utf-8 -*-
+// Copyright (C) 2017 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 3 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 this program.  If not, see <http://www.gnu.org/licenses/>.
+
+#pragma once
+
+enum color_type { color_never, color_always, color_if_tty };
+
+extern color_type color_opt;
+extern const char* bright_red;
+extern const char* bold;
+extern const char* bold_std;
+extern const char* reset_color;
+
+extern const struct argp color_argp;
+void setup_color();