[cpp-threads] Draft Plan for Threads in C++

[Peter Dimov]

Boehm, Hans wrote:
> I would actually lean towards "always implemented, with feature tests
> to determine whether it's an emulation".

My vote goes to "implementation defined if unimplementable in a lock-free 
manner on the platform."

I'm not sure whether we should acknowledge one possible emulation strategy 
that uses the variable being manipulated as a spinlock by reserving a 
sentinel value.