Improve QHull finder script
Improve QHull finder script

* Make sure QHULL_LIBRARIES and QHULL_INCLUDE_DIRS variables are not
  defined if QHull is not found
* Make QHULL_LIBRARIES and QHULL_INCLUDE_DIRS advanced options
* Some reformatting

drmateo/pcl,drmateo/pcl,drmateo/pcl,drmateo/pcl,drmateo/pcl