Currently, we are enforcing the _FORTIFY_SOURCE=2 without any
previous detection if the macro has been already defined, e.g.
by environment, or is just enabled by compiler by default.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Jan Vesely <jano.vesely@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>