maint: update to latest gnulib

maint: update to latest gnulib

Upstream gnulib determined that we were needlessly compiling in
gnulib's regex instead of glibc's when targetting new-enough glibc,
because the m4 test was being too strict in requiring a particular
answer to undefined behavior.
https://lists.gnu.org/archive/html/bug-gnulib/2013-04/msg00032.html

* .gnulib: Update to latest, for regex.
(cherry picked from commit 842432390b742193c89f6b6f9991bc7ceea8d836)

Cherry picked because I hit a failure of test-regex when swapping
between incremental builds of libvirt.git master then v1.0.4-maint.