maint: update to latest gnulib

While we aren't impacted by most of upstream's changes this
month, at least the regex improvement looks useful.

* gnulib: Synchronize to upstream.

.gnulib @ ce457d62

-Subproject commit 72fb9075b2812765d2ce55a1a26c3764392377cb
+Subproject commit ce457d622d947d8aa3e325d98a2818d097e7fec9