Hm okay. I just tested with
curl -LO https://www.vtk.org/files/release/9.0/VTK-9.0.1.tar.gz
tar xf VTK-9.0.1.tar.gz
cd VTK-9.0.1/
mkdir build
cd build
cmake -DVTK_MODULE_ENABLE_VTK_RenderingContextOpenGL2=YES ..
and I’m not getting that warning here. Full transcript here: transcript.txt (30.0 KB)
Maybe it has something to with what groups/modules get enabled by default on your system. Would help to see the output from the CMake run.