A monotonic clock is necessary to accurately calculate time passed. clock_gettime is one option, but only works for linux and BSD. (afaik)
The system call was introduced in the 2.6 kernel, but the wrapper only in glibc 2.17. Which means we have to dynamically link against librt to get a wrapper, adding a runtime dependency on libdavix, and by extension on the ROOT plugin, and by extension on ROOT itself. Not good.
1. Live without a monotonic clock, relying on gettimeofday (current option)
2. Try to use std::chrono::steady_clock if available, fallback on issuing the syscall directly without a wrapper, fallback on gettimeofday