Remove hack to append "-o a.out" when we actually don't want to build any executeble.

This behaviour
  * caused parmake / ghc to compile with only one thread in cetain
    situations
  * hid the bug that using only ghc -M and then ghc -c per module,
    parmake *cannot* detect whether an external (e.g. used package)
    module (interface file) changed when doing an incremental build
    (ghc itself can detect that).
    We will have to detect this by inspecting .hi files, like ghc does.