gnulib: update to latest upstream revision.

* gnulib: Update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
version 2.4.2.418

* NEWS: Record release date.
maint: post-release administrivia

* NEWS: Add header line for next release.
* .prev-version: Record previous version.
* cfg.mk (old_NEWS_hash): Auto-update.