build: update gnulib to latest to fix MacOS X 10.5 test link failure

Details here:
http://thread.gmane.org/gmane.comp.gnu.coreutils.general/1726/focus=1743

diff --git a/gnulib b/gnulib
index d813b688732c3a0da947f91cbb19cb78a627209e..6a4c64ce4a59bd9589e63fb5ee480765d356f8c7 160000 (submodule)
--- a/gnulib
+++ b/gnulib
@@ -1 +1 @@
-Subproject commit d813b688732c3a0da947f91cbb19cb78a627209e
+Subproject commit 6a4c64ce4a59bd9589e63fb5ee480765d356f8c7