code: public void addConfiguredLinker(final LinkerDef linker) {
      if (linker == null) {
        throw new NullPointerException("linker");
      }
      linker.setProject(getProject());
      this._linkers.addElement(linker);
    }
docs: Adds a linker definition. The first linker that is not disqualified by
  its "if" and "unless" attributes will perform the link. If no child
  linker element is active, the linker implied by the cc elements name or
  classname attribute will be used.
  
  @param linker
  linker
  @throws NullPointerException
  if linker is null