configure: Define _FORTIFY_SOURCE conditionally
Firstly, some build environments may already define
_FORTIFY_SOURCE so if we put it straight onto command line we're
redefining it and that makes compiler complain. Secondly,
_FORTIFY_SOURCE is not allowed without optimizations enabled.
At the same time, enable lint to follow the lead of coreutils for
using improved glibc headers.
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>