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

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.