rendernode example builds incorrectly for D3D when out of tree

The new rendernode example in quick/scenegraph contains optional code that is guarded by

config_d3d12 { ... }

in the .pro. This works in in-tree example builds but apparently fails to trigger when building the example as shipped in the official packages (since the module-specific .qmake.cache with CONFIG+=config_d3d12 is gone there).

The result is an executable that crashes when running on D3D12 due to not having the necessary code paths compiled in.

Switching to qtConfig(d3d12) does not work since d3d12 is not a feature in qtbase.

How can an example test for the success of a module-specific config test?