In Qt >= 5.12, the qhelpgenerator and qcollectiongenerator tools were fused, only qhelpgenerator remains.
Allow using ECMQueryQmake inside ECM.
Due to project(ECM) specifying the "NONE" language, find_package(Qt5Core)
cannot succeed.
if the Qt5::qmake target doesn't exist, we now look for the qmake executable.