Ok, it works now. I don’t really know what fixed it now though. I disabled hyperthreading and built it with 2 processors, but still no success. With system clang the build crashed before UE4Editor. I tried it again with bundled clang, still those errors. However, I did another “make” then and the 68 failed steps could be compiled. Starting the editor resulted in a crash again, but was successful after a reboot. The Editor is now running. Yet I’m very afraid of applying any updates and having to rebuild it 
Anyway, thanks libhack! Although I’m not sure if it was only cause of hyperthreading.