bootstrap: sync with upstream.

* build-aux/bootstrap.in (func_require_gnulib_submodule): Add
gnulib_path argument to prevent updating all submodules.
* build-aux/boostrap.slingshot (func_require_slingshot_submodule):
Likewise for slingshot submodule updates.
* bootstrap: Regenerate.
* NEWS: Update.
Reported by Reuben Thomas

Signed-off-by: Gary V. Vaughan <gary@gnu.org>