Rock.cmake: fix some comments

- doubled "exists" in one sentence
- libraries are not automatically linked against QtCore (they are not?)

this is an older commit I stumpled upon in my workdir... just not want to throw it away

Signed-off-by: Martin Zenzes <martin.zenzes@dfki.de>
Well, whether what we need to change is the documentation, or if we should change to code to match the documentation. I am not so sure myself.