Fix wall-clock time calculation: use `clock_gettime` instead of `gettimeofday`

In the case of `gettimeofday`, inaccuracies may occur when the system clock is adjusted due to NTP or leap second handling. As a result, calculating the difference between two time points can produce incorrect values. Issue #178 was related to this: for very short execution times, the `time-wall` value occasionally appeared as negative. Therefore, the method of calculating the time difference has been changed from `gettimeofday` to `clock_gettime`. For more details, please refer to the commit history. :) Close #178 See also: [POSIX.1-2008 specification for gettimeofday (obsolescent function)](https://pubs.opengroup.org/onlinepubs/9699919799/functions/gettimeofday.html)