Support icc/icpc

[wjmelements]

icc and icpc are the required compilers on several clusters including TACC Stampede, and I've had to make some adjustments every time I've tried to get it to compile.

Basic changes:
cds/cxx11_atomic.h needs to detect that icpc has the atomic header
icpc is missing certain features of the atomic header. This is a shortlist:

• std::atomic_ typedefs
• atomic_memory_fence and similar functions are not in std::

If you think this is a worthy feature I may have time to do it properly the week after next.

[wjmelements]

The reason to prefer the Intel headers to the cds version is that the compiler might have optimizations in mind that it only applies to those definitions.
I'm sufficiently satisfied by the performance of the default that I have closed this for now.