configure: Undefine _FORTIFY_SOURCE prior using it 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>