LibreSSL checks the size of time_t while building (configure or cmake) for year 2038 issue, and shows warning if the size of time_t is 4.

Warning, this system is unable to represent times past 2038 It will behave incorrectly when handling valid RFC5280 dates

The check is at c8918d.

Check if time_t is sized correctly AC_CHECK_SIZEOF([time_t], [time.h])