Instead of using ninja/make directly, there is a cmake command option to
trigger the build using whatever build tool has been configured. Use it;
if there is a mismatch, there will also be a clearer cmake error.

Remove build_oai's --build-coverity: we don't use it, and it can
trivially be added if required (probably, most users would run it by
hand anyway).

Add an error if the compilations function is not called with the right
noumber of arguments.