add_flag doesn't work well with std::atomic<bool>

[phlptp]

ran into an issue with the add_flag not handling atomic properly.
It seems to trigger a deleted assignment operator in atomic.

I made a workaround in the application for now but I will be looking into this soon.

[phlptp]

The workaround was to use a add_flag_function

group->add_flag_function("--flush",
                            [this](int64_t val){
        if (val > 0) {
            forceLoggingFlush = true;
        }},

forceLoggingFlush is an atomic_bool member