CMake doesn't provide the equivalent of QT_INSTALL_QML

CMake for Qt4 provided a QT_IMPORTS_DIR pointing to the folder where default QML modules were installed.

For Qt5 there is no longer such a variable, and there's nothing else that provides an equivalent of qmake's QT_INSTALL_QML.

QT_INSTALL_QML is the path to where globally-available QML modules should be installed. For CMake to support installing QML modules in that path, it (or an equivalent) has to be available from one of the modules (Qt5Quick, probably?).