clock_gettime is not portable. gethrxtime takes the best available
option to get the wall clock time, including clock_gettime (monotonic
clock), and gettime (non monotonic).
Also, using xtime_t instead of float preserves the precision.
Suggested by Bruno Haible.
* lib/xtime.h (xtime_make): Handle overflows of nanoseconds.
* modules/timevar (Depends-on): We need gethrxtime.
We no longer use times().
(Link): Update.
* lib/timevar.h (timevar_time_def): Use xtime_t.
* lib/timevar.c (set_to_current_time): Use gethrxtime.
(timevar_print): Instead of checking whether the timings themselves
are large enough for the timevar to be printed, check the percentages.