maint: automate annual package-wide copyright-year update.

* .x-update-copyright: New.
* Makefile.am (EXTRA_DIST): Remove maint.mk.
* bootstrap.conf (gnulib_modules): Add maintainer-makefile and
update-copyright.  Remove gnumakefile, which is implied by
maintainer-makefile.
* cfg.mk (bootstrap-tools): Copy from old maint.mk.
* gnulib: Update.
* maint.mk: Remove, now copied from gnulib.
* examples/extexi: Add missing "(C)" in copyright statement so
update-copyright can recognize it.
* src/LR0.h: Likewise.
* src/print.h: Likewise.
* src/print_graph.h: Likewise.
* src/gram.c: Add missing comma in copyright statement.
* src/gram.h: Likewise.

.gitignore

@@ -26,6 +26,7 @@
 /gnulib
 /javacomp.sh
 /javaexec.sh
+/maint.mk
 /patches
 /releases
 /stamp-h*