cmake: Cannot find Qt5 on SunOS / illumos (OpenIndiana Distribution)

[hebasto]

The Qt5 was installed as follows:

$ sudo pkg install qt5

As a workaround, the Qt5_DIR variable can be provided:

$ cmake -B build -DBUILD_GUI=ON -DQt5_DIR=/usr/lib/qt/5.15/lib/amd64/cmake/Qt5