maint: update to latest gnulib, for older autoconf

Based on a report from Pavel Hrdina, gnulib was fixed to support
AC_PROG_SED even when using ancient autoconf 2.59 of RHEL 5.

* .gnulib: Update to latest, to fix build on RHEL 5.
Signed-off-by: NEric Blake <eblake@redhat.com>

.gnulib @ 72fb9075

-Subproject commit 3f51bf41c8be8b310f57caff371377414701d5cc
+Subproject commit 72fb9075b2812765d2ce55a1a26c3764392377cb