ltlcross: allow appending to files via >>FILENAME

Suggested by Joachim Klein.  Fixes #57.

* src/bin/common_file.hh, src/bin/common_file.cc: New file.
* src/bin/Makefile.am: Build them.
* src/bin/ltlcross.cc: Use the output_file class.
* src/tgbatest/ltlcross3.test: Test >>.
* NEWS: mention it.

src/bin/common_file.hh

@@ -0,0 +1,54 @@
+// -*- coding: utf-8 -*-
+// Copyright (C) 2015 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/>.
+
+#ifndef SPOT_BIN_COMMON_FILE_HH
+#define SPOT_BIN_COMMON_FILE_HH
+
+#include "common_sys.hh"
+#include <iosfwd>
+#include <fstream>
+
+class output_file
+{
+  std::ostream* os_;
+  std::ofstream* of_ = nullptr;
+  bool append_ = false;
+public:
+  // Open a file for output.  "-" is interpreted as stdout.
+  // Names that start with ">>" are opened for append.
+  // The function calls error() on... error.
+  output_file(const char* name);
+
+  ~output_file()
+  {
+    delete of_;
+  }
+
+  bool append() const
+  {
+    return append_;
+  }
+
+  std::ostream& ostream()
+  {
+    return *os_;
+  }
+};
+
+#endif // SPOT_BIN_COMMON_FILE_HH