Prefer MK_SSP=no to SSP_CFLAGS=
It is more idiomatic. CFLAGS is only augmented with $SSP_CFLAGS when
$MK_SSP != "no".
Reviewed by: imp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D31401
(cherry picked from commit 61ed578ee66648eff206ee5622b66727bfb52e78)