It looks like there has been an update to Galera that has broken the the build process. The errors suggest it's missing the same libatomic library as MariaDB.

A quick stab in the dark would be to add the following line:

list(APPEND GALERA_UNIT_TEST_LIBS atomic)

On line 26 in the cmake/check.cmake file:

https://github.com/MariaDB/galera/blob/mariadb-4.x/cmake/check.cmake#L26

Then rebuild from scratch by using the scratch command:

./scripts/build.sh --scratch -p -j 1

If that doesn't work then it would need further investigation which I will try and do when I have some spare time,