Rock.cmake: fix some comments

- originally: remove a doubled "exists" in one sentence
- libraries are not automatically linked against QtCore (they are not?)
- corrected the path where vizkit is supposed to look at stuff

Signed-off-by: Martin Zenzes <martin.zenzes@dfki.de>
Merge pull request #6 from marvin2k/reword_comments

Rock.cmake: fix some comments