For the first time it looks like the atomic library is not being used for unit tests. There are four instances of it being used within gcs/src/gcs.cpp but for some reason the GCS (Group Communication System) library seemed to build correctly and it wasn't until the GARB (Galera Arbitrator) library referenced it that it failed.

My best guess at the moment would be to add the atomic library when it builds the gcs.cpp file. This means modifying line 51 of gcs/src/CMakeLists.txt

https://github.com/MariaDB/galera/blob/mariadb-4.x/gcs/src/CMakeLists.txt#L51

So it becomes:

target_link_libraries(gcs gcomm gcache atomic)

If that doesn't work then we might have to add the atomic library whenever the GCS library is referenced throughout the Galera project.