build: update to latest gnulib

build: update to latest gnulib

Gnulib finally relaxed the isatty license, needed as first mentioned here:
https://www.redhat.com/archives/libvir-list/2012-February/msg01022.html

Other improvements include better syntax-check rules (we can delete one
of ours now that it is a duplicate) and better compiler warning usage.

* .gnulib: Update to latest, for isatty.
* cfg.mk (sc_prohibit_strncpy): Drop a now-redundant rule.
* bootstrap.conf (gnulib_modules): Add isatty.
* bootstrap: Resync from gnulib.