build: update to a usable gnulib commit

The commit, 5ef90695, 2009-07-29, "maint: move update-copyright
to gnulib" recorded a local-only commit.
Reported by Pádraig Brady.

diff --git a/gnulib b/gnulib
index 29a35cf5baff3fa063a995e91c28986859ad865f..bc38a4c491577399f8fae0a8138e5437249d50b3 160000 (submodule)
--- a/gnulib
+++ b/gnulib
@@ -1 +1 @@
-Subproject commit 29a35cf5baff3fa063a995e91c28986859ad865f
+Subproject commit bc38a4c491577399f8fae0a8138e5437249d50b3