attempt to default enable stack-protector-strong

Change-Id: Iee42b3f6ab66636961e9ba1da33ab0e83d607eaa
Reviewed-on: https://gerrit.libreoffice.org/13241
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>