build: update gnulib to latest, to avoid FreeBSD compilation failure:

http://thread.gmane.org/gmane.comp.lib.gnulib.bugs/23501

diff --git a/gnulib b/gnulib
index c823199df2cc03b6bd70d0a2fef5999af82792fe..a4c6c61e4b224834f9b6a51fce16f3adbe1d982e 160000 (submodule)
--- a/gnulib
+++ b/gnulib
@@ -1 +1 @@
-Subproject commit c823199df2cc03b6bd70d0a2fef5999af82792fe
+Subproject commit a4c6c61e4b224834f9b6a51fce16f3adbe1d982e