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/getargs.c

@@ -22,6 +22,7 @@
 
 #include <config.h>
 #include "system.h"
+#include "revision.h"
 
 #include <argmatch.h>
 #include <error.h>
@@ -272,6 +273,7 @@ version (void)
      continue.  */
   printf (_("bison (GNU Bison) %s"), VERSION);
   putc ('\n', stdout);
+  printf ("%s", revision);
   fputs (_("Written by Robert Corbett and Richard Stallman.\n"), stdout);
   putc ('\n', stdout);