The -n switch is not strictly a part of the Build program. Instead, it is just passed to the make(1) program, and the Build program creates the obj directory (if it doesn't exist), and then populates it with a Makefile and symbolic links. After that, Build invokes make with the -n switch, which only causes make to print what it would do. It doesn't cause make to actually do anything.

This -n switch is especially useful with the install target, to preview what steps will be undertaken to install the program before actually installing it.