Between Bison releases, manually append `+' to the previous Bison

release number, and use that as a signal to automatically print the
ChangeLog's CVS Id keyword from --version.  Discussed starting at
<http://lists.gnu.org/archive/html/bison-patches/2006-05/msg00028.html>.
* ChangeLog: Add Id header.
* configure.ac (AC_INIT): Append `+' to `2.3'.
* src/.cvsignore: Add revision.c.
* src/Makefile.am (bison_SOURCES): Add revision.c and revision.h.
(BUILT_SOURCES): Add revision.c.
(revision.c): New target rule.  This file defines a new global variable
named revision.  It initializes it with either the Id from ChangeLog
or, if VERSION doesn't contain `+', with the empty string.
* src/getargs.c (version): Print the value of revision.
* src/revision.h: Extern revision.

src/revision.h

@@ -0,0 +1,27 @@
+/* Between Bison releases, define a precise revision string.
+
+   Copyright (C) 2006 Free Software Foundation, Inc.
+
+   This file is part of Bison, the GNU Compiler Compiler.
+
+   Bison 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, or (at your option)
+   any later version.
+
+   Bison 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 Bison; see the file COPYING.  If not, write to the Free
+   Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+   02110-1301, USA.  */
+
+#ifndef REVISION_H_
+# define REVISION_H_
+
+extern const char *revision;
+
+#endif /* !REVISION_H_ */