when -fstack-protector-strong is not available, like it was implicitly before
r288669
As noted by antoine@, devel/gcc (which is 4.8.5) lacks -fstack-protector-strong
support, whereas 4.8.4i (devel/gcc48) has the support.
Until a version is available which has -fstack-protector-strong support, be
conservative and only enable support with 4.9+.
Reviewed by: pfg
X-MFC with: r288669, r289465
Differential Revision: https://reviews.freebsd.org/D3924
a52bef9c99