Take care of system clock change

[kenmasumitsu]

spdlog uses std::condition_variable. However std::condition_variable does not use monotonic timer.
It means that when system clock is changed, it does not work expectedly.

One solution is using pthread with pthread_condattr_setclock(&m_attr, CLOCK_MONOTONIC); instead of std::thread.
https://stackoverflow.com/questions/51005267/how-do-i-deal-with-the-system-clock-changing-while-waiting-on-a-stdcondition-v

[gabime]

This is a well known issue in the libstd++/glibc implementation of std::condition_variable.

I don't think spdlog should try to fix such fundamental thing by its own.