When CMake uses ninja as backend instead of UNIX Makefiles, the building process fails because it cannot download Z3.

The Z3 download process is configured as:

It is a known issue, that countermeasures has to be made due to ninja's dependency scanning mechanism: https://stackoverflow.com/questions/50400592/using-an-externalproject-download-step-with-ninja

This issue should either be resolved, or marked as wontfix, and the documentation extended accordingly.