Merge branch '21.0-buildfixes' into '21.0'

Take CMake modules functions that build from git properly

Note that this is a temporary fix, pending the switch
to storing the ATLAS CMake helpers in the AtlasExternals
repository (which @akraszna is doing)

However, it should be accepted quite urgently as at the
moment the cmake step on the 21.0 branch fails, so no
builds are possible.

See merge request !163