Support GCC/LLVM stack-smashing protection

Turn it on with `cmake -DENABLE_SSP=1` Protection buffer/stack overflow exploits.

Starting with version GCC 4.9 there is support for more advanced protection from overflows [`-fstack-protector-strong`](http://goo.gl/OPIZ4Y) (minor loss of performance), the version below only support [`-fstack-protector`](http://goo.gl/OPIZ4Y).
LLVM 3.6 supports [`-fstack-protector-strong`](http://goo.gl/OPIZ4Y) prior to this worked as a stripped-down option [`-fstack-protector`](http://goo.gl/OPIZ4Y).

- [RELRO: RELocation Read-Only](https://isisblogs.poly.edu/2011/06/01/relro-relocation-read-only/)