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.

diff --git a/.gnulib b/.gnulib
index 076ac82d1d7f4df54630f1b4917b3c14f227f032..92f3a4c8e52e64c233e260431d095dbf88554c14 160000 (submodule)
--- a/.gnulib
+++ b/.gnulib
@@ -1 +1 @@
-Subproject commit 076ac82d1d7f4df54630f1b4917b3c14f227f032
+Subproject commit 92f3a4c8e52e64c233e260431d095dbf88554c14