Improve copyright updating.
* build-aux/update-copyright: Resynchronize from upstream.
* maint.mk (update-copyright): Simplify based on gnulib.
(update-copyright-env): New variable.
* cfg.mk (update-copyright-exclude-regexp): Delete.
(update-copyright-env): New override.
* .x-update-copyright: New file.
* lib/Autom4te/Makefile.am: Add copyright.
* lib/Autom4te/Channels.pm: Revert copyright update to upstream
file.
* lib/Autom4te/Configure_ac.pm: Likewise.
* lib/Autom4te/FileUtils.pm: Likewise.
* lib/Autom4te/Struct.pm: Likewise.
* lib/Autom4te/XFile.pm: Likewise.
Signed-off-by: Eric Blake <ebb9@byu.net>