maint: update to latest gnulib

* gnulib: Update to latest, to avoid warnings with latest autoconf.git.

diff --git a/gnulib b/gnulib
index 91fd3b18b755b77c4bb3f7556ddedb055c16c834..a228f539d716ba1120173d7d5bf96bfbdc5e63aa 160000 (submodule)
--- a/gnulib
+++ b/gnulib
@@ -1 +1 @@
-Subproject commit 91fd3b18b755b77c4bb3f7556ddedb055c16c834
+Subproject commit a228f539d716ba1120173d7d5bf96bfbdc5e63aa