SNTP_COMP_ROUNDTRIP macro

Enable round-trip delay compensation. Compensate for the round-trip delay by calculating the clock offset from the originate, receive, transmit and destination timestamps, as per RFC. The calculation requires compiler support for 64-bit integers. Also, either SNTP_SET_SYSTEM_TIME_US or SNTP_SET_SYSTEM_TIME_NTP has to be implemented for setting the system clock with sub-second precision. Likewise, either SNTP_GET_SYSTEM_TIME or SNTP_GET_SYSTEM_TIME_NTP needs to be implemented with sub-second precision. Although not strictly required, it makes sense to combine this option with SNTP_CHECK_RESPONSE >= 2 for sanity-checking of the received timestamps. Also, in order for the round-trip calculation to work, the difference between the local clock and the NTP server clock must not be larger than about 34 years. If that limit is exceeded, the implementation will fall back to setting the clock without compensation. In order to ensure that the local clock is always within the permitted range for compensation, even at first try, it may be necessary to store at least the current year in non-volatile memory.