With cmake, how would you disable in-source builds?

Question

I want to disallow people from cluttering our source tree with generated CMake files... and, more importantly, disallow them from stepping on existing Makefiles

Answer 1

Just make the directory read-only by the people/processes doing the builds. Have a separate process that checks out to the directory from source control (you are using source control, right?), then makes it read-only.