Only enable -fstack-protector-strong on gcc 4.9+ and default to -fstack-protector

Only enable -fstack-protector-strong on gcc 4.9+ and default to -fstack-protector
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