BUG: Fix launcher settings generation with system qt.
First, it sets Slicer_USE_SYSTEM_QT=ON in the case of Qt5.
And also fix appending QT_BINARY_DIR in the case of the system qt5. (this was generating problems with system python and virtualenvs)
Co-authored-by: Jean-Christophe Fillion-Robin JChris.FillionR@kitware.com