Upstream gnulib recently patched a bug in bootstrap, for projects
that use a different name than build-aux for a subdirectory.  We
don't, but it doesn't hurt to update.

* .gnulib: Update, for bootstrap fix.
* bootstrap: Sync to upstream.
* bootstrap.conf: Match upstream bug fix.
Signed-off-by: NEric Blake <eblake@redhat.com>
(cherry picked from commit ac0852c7)