configure and clock_gettime for macOS

[Peter Balck]

Hi Jef
Found a problem regarding the timer.c ifdefs in the dc_timer_new function.. 
When I build libdivecomputer on my mac running the latest version of macOS the configure script will find the clock_gettime function and define HAVE_CLOCK_GETTIME. This however, will cause anyone running my app on an older version of macOS where clock_gettime is not available to crash. 
Is there an easy way to override the configure script without changing any of the source or config files ? 

I also found that logs downloaded from the Mares Smart Apnea have the max and min temperature mixed up. 

Thanks
Peter