Added detection of -fstack-protector-strong in compiler, and if
available (GCC >= 4.9, clang >= 3.5) turn it on. Costs according to
the Chrome team who implemented it is about a 3% increase in stack
consumption. Apparently they're turning it on in the Linux kernel
by default, which is good enough for me.

Removed -fPIC and -pie from clang builds with tsan on. Had hoped
from the clang sources it would work around the warnings about
infinite stack, but it doesn't.