[cpp-threads] Update

[Peter Dimov]

Boehm, Hans wrote:

>> This formulation supercedes both strong CAS and weak CAS, as
>> it's allowed to fail. A strong CAS is easily built on top of
>> it by using a retry loop.

> You're assuming the feature test for whether this is wait-free will
> yield false if it can fail spuriously?  Otherwise you can't build
> guaranteed wait-free algorithms?  Or do you not care?  (This is a
> serious question:  I'm not sure if I do.)

I can't think of a reason to ever use that feature test. What could I do 
with the answer?