clean paths of found files in prepareProject()

any of the directories may be the root, which would lead to double slashes
in the constructed filenames.

Change-Id: I053e167a19b795b40e780fc29db356c7f24d286a
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
(cherry picked from qtbase/97be79012cd39d1d0e7748d5c500d86c3becc258)