Only use -fstack-protector-strong with supported compilers

This includes clang 3.5.0+, gcc 4.2.1, gcc 4.8.4+

This allows me to do subdirectory makes again after setting
MAKESYSPATH on 10.2-RELEASE as it comes with clang 3.4.1.

As a sidenote: this isn't technically correct for all vintages
of gcc 4.2.1, but will be correct when gcc is rebuilt/reinstalled
after r286074, so this version check should be good enough.

X-MFC with: r288669
Differential Revision: https://reviews.freebsd.org/D3924
Reviewed by: emaste, pfg