Thread::fShouldExit should of type std::atomic<bool>

[relascope]

AFAIK the C++ keyword volatile has to do with compiler optimization (especially ordering). (it was a little different in C)
std::atomic guarantees "undefined behaviour"-free code, when multiple threads access a std::atomic<trivialtype> variable concurrently like it is possible in Thread through internal running Thread and concurrent other threads through the public Thread::signalThreadShouldExit.

[falkTX]

C++98 does not have std::atomic, which DPF still targets as minimum requirement.

I do not want to introduce new C++ requirements until very strictly necessary.
The volatile there is meant more as an indication that it should never be optimized away, as a precaution more than reassurance.