I'm getting a warning in the file boost/thread/pthread/timespec.hpp pointing out that clock_gettime is only available on macOS 10.12 and later. This code is being compiled on macOS 10.13, but should be compatible with macOS versions back to 10.7. How can I fix it? Maybe change the setting of BOOST_THREAD_USEFIXES_TIMESPEC, which I don't see any documentation for?