build: update gnulib submodule; also bootstrap to latest

Notes tests/init.sh is still in sync with gnulib.

* bootstrap: Update to latest.
* gnulib: Update avoiding gnulib-test failures.
doc: mention prior stat change

For a file of size 1234 bytes, commit ca9aa759 had the side effect
of changing 'stat -c "%'s" file' from outputting "?s" to the nicer
"1,234", depending on locale.  This is worth mentioning in the NEWS.

Resolves part of http://bugs.gnu.org/14556.

* NEWS: Mention 8.7 improvement in stat.
* cfg.mk (old_NEWS_hash): Adjust accordingly.