This is a sibling of atlasexternals!595 (merged). It only works with the code proposed in that MR. (Hence it's a WIP, as it wouldn't work as-is. I have additional changes locally, which make sure that my test build uses https://gitlab.cern.ch/akraszna/atlasexternals/tree/CMakeUpdates-master-20200106.
As described in atlasexternals!595 (merged) and in ATLINFR-3157, we need to make some changes to be compatible with CMake 3.15+. These are the proposed changes.
The most important change is that atlas_project(...) no longer receives the name and version of a project. Those now have to be specified in a call to project(...) at the top of the project CMakeLists.txt files. However the version numbers, in practice, are still read in from the same version.txt files as before. So for people not regularly editing the project CMakeLists.txt files, this should not cause any difference.
Most of the documentation / discussion about these updates is in atlasexternals!595 (merged).