When Qt 5 is not found CMake suggests to either add an appropriate path to CMAKE_PREFIX_PATH, or to set the Qt5_DIR cache variable. Currently we only support the former variant. However, the past weeks revealed that almost all (internal) MITK users were puzzled as they only tried the non-working latter variant. Hence, we clearly need to support Qt5_DIR as well.
To keep changes to our build system as minimal as possible we should just derive a path from Qt5_DIR (only if set) which we append to CMAKE_PREFIX_PATH (only if not already a list item).