I just configured with this cmake line:
cmake -DCMAKE_OSX_DEPLOYMENT_TARGET:STRING=10.15 -DCMAKE_BUILD_TYPE:STRING=Debug -DQt5_DIR:PATH=/usr/local/Cellar/qt/5.15.2/lib/cmake/Qt5 ../Slicer
and it’s building now. Should work, I’ll let you know if it doesn’t.
Even if you’re making a custom module, why do you need to build Qt from source?