maint: update to latest gnulib

Several portability changes, but the one we are most interested in
is the improvement to bootstrap to detect perl modules.

This patch doesn't actually change our bootstrap requirements
(that will be a separate patch), but sets the stage for it.

* .gnulib: Update to latest.
* bootstrap: Regenerate from upstream.

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