build: update to latest gnulib

In particular, pull in gnulib's fix for the broken AC_HEADER_MAJOR
macro in autoconf.

Signed-off-by: Eric Blake <eblake@redhat.com>

diff --git a/.gnulib b/.gnulib
index a2a39436b65f329630df4a93ec4e30aeae403c54..e89b4a7aefce9cb02963920712ba7cdd13641644 160000 (submodule)
--- a/.gnulib
+++ b/.gnulib
@@ -1 +1 @@
-Subproject commit a2a39436b65f329630df4a93ec4e30aeae403c54
+Subproject commit e89b4a7aefce9cb02963920712ba7cdd13641644