build: update to latest gnulib, for secure tarball

build: update to latest gnulib, for secure tarball

Pick up some build fixes in the latest gnulib.  In particular,
we want to ensure that official tarballs are secure, but don't
want to penalize people who don't run 'make dist', since fixed
automake still hasn't hit common platforms like Fedora 17.

* .gnulib: Update to latest, for Automake CVE-2012-3386 detection.
* bootstrap: Resync from gnulib.
* bootstrap.conf (gnulib_extra_files): Drop missing, since gnulib
has dropped it in favor of Automake's version.
* cfg.mk (local-checks-to-skip): Conditionally skip the security
check in cases where it doesn't matter.