Eliminate distinction between `prefix` and `install-dir`
They mean the same thing to kdesrc-build and while we may support both on the user side for compatibility, we can reduce down to only one way to express "install dir" within the code.
They mean the same thing to kdesrc-build and while we may support both on the user side for compatibility, we can reduce down to only one way to express "install dir" within the code.