Bug 18113 small code improvements

A few more small improvements to add.
http://bugs.mitk.org/show_bug.cgi?id=18113
I think it will not work in the standard case and would never lead to the inclusion of the ToF install rules file. Thanks for clarifying - in your case the file is never created in the build directory, so I would just add a `if(EXISTS ...)` statement instead of `if(mitkToFHardware_IS_ENABLED)`.