Replace MITK_USE_QT with MITK_USE_Qt5
MITK_USE_QT_WEBENGINE is replaced with MITK_USE_Qt5_WebEngine.
Conditional blocks of our build script rely on a MITK_USE_${package} variable. By changing MITK_USE_QT to MITK_USE_Qt5, these blocks function as they are expected to do. For example, MITK can be built without Qt again.