Add a new library to generate formulas and automata.

This library, called libspotgen, gathers functions to generate classes
of automata found in the literature.
Related to #254.

* NEWS, README: Mention the modification.
* Makefile.am, debian/control, debian/libspotgen0.install: Build the new
  library in a separate package.
* spot/gen/automata.hh, spot/gen/automata.cc: Add a family of co-Büchi
  automata.
* configure.ac, spot/Makefile.am, spot/gen/Makefile.am: Build the new
  library.

spot/gen/Makefile.am

@@ -0,0 +1,30 @@
+## -*- 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/>.
+
+AM_CPPFLAGS = -I$(top_builddir) -I$(top_srcdir) $(BUDDY_CPPFLAGS)
+AM_CXXFLAGS = $(WARNING_CXXFLAGS)
+
+spotgendir = $(pkgincludedir)/gen
+spotgen_HEADERS = automata.hh
+
+lib_LTLIBRARIES = libspotgen.la
+libspotgen_la_SOURCES = \
+  automata.cc
+#libspotgen_la_LDFLAGS = $(BUDDY_LDFLAGS) -no-undefined $(SYMBOLIC_LDFLAGS)
+