make clang tooling optional

I am in a docker with clang 10 and configure fails for me since its the wrong version. However, this is a docker where I just want to compile some stuff and am not interested in clang format, analyze or tidy. I would prefer if I only get a warning, not an error. From a quick glance at our cmake macros, a warning (instead of an error) is indeed what we intend, though not what happens currently...