this fixes a linking problem with RPi 3 (and probably others) running
with Raspbian (new Raspberry Pi OS) that can't use the inline
atomic functions but instead require linking against the lib -latomic.
The CMake code is based on SoapyRTLSdr file (licensed under MIT)
https://github.com/pothosware/SoapyRTLSDR/blob/master/CheckAtomic.cmake