maint: update to latest gnulib

maint: update to latest gnulib

Since we're about to freeze, it's time to pick up the latest
upstream gnulib.  Among other changes, gnulib now guarantees the
use of some -f flags that we were previously manually adding.

* .gnulib: Update to latest, in part for warning improvements.
* m4/virt-compile-warnings.m4 (LIBVIRT_COMPILE_WARNINGS): Drop
flags that are now guaranteed by gnulib.
* bootstrap: Resync to gnulib.

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