Add support for "-fstack-protector-strong". This extends the
WITH_SSP_PORTS Makefile option in FreeBSD which adds "-fstack-protector"
and "-fstack-protector-all" command-line options that add extra code to
check for buffer overflows to ports built that way, cf.
https://gcc.gnu.org/onlinedocs/gcc-4.8.3/gcc/Optimize-Options.html
While this was a good first step, those switches offer too little
protection or too much overhead and so Google contributed a balanced
"-fstack-protector-strong". [1]
PR: 186852 [1]
Submitted by: software-freebsd@interfasys.ch [1]
