it may be present but not implemented. Add sudo_gettime_real() and
sudo_gettime_mono() functions to get the real and monotonic times
respectively. Now sudo_gettime_mono() checks the value of
sysconf(_SC_MONOTONIC_CLOCK) before calling clock_gettime(CLOCK_MONOTONIC)
and falls back on sudo_gettime_real() as needed. The Mach version
of sudo_gettime_mono() uses mach_absolute_time().
This should fix problems with timestamp files on systems where
the CLOCK_MONOTONIC is defined but not actually implemented.
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