gnulib: update to latest, for getprogname fixes

Compiling in gnulib-tests/ would fail on some platforms
due to a missing lib/assert.h.  Reported by Assaf Gordon in
https://lists.gnu.org/archive/html/coreutils/2016-09/msg00016.html

diff --git a/gnulib b/gnulib
index 7da1068a87c331793af73359d8637d140d08a674..6d237fd01d98f2cdd588055a088fed39646ab82f 160000 (submodule)
--- a/gnulib
+++ b/gnulib
@@ -1 +1 @@
-Subproject commit 7da1068a87c331793af73359d8637d140d08a674
+Subproject commit 6d237fd01d98f2cdd588055a088fed39646ab82f