Those final compilation parameter issues. Removed unused include.

install.sh

@@ -27,7 +27,7 @@ cd VTK-9.0.1
 mkdir build
 cd build
 export ospray_DIR=../../ospray-1.8.5/lib/cmake/ospray-1.8.5
-../../cmake-3.19.4/bin/cmake .. -DVTK_MODULE_ENABLE_VTK_RenderingRayTracing=YES
+../../cmake-3.19.4/bin/cmake .. -DVTK_MODULE_ENABLE_VTK_RenderingRayTracing=YES -DVTK_OPENGL_HAS_OSMESA=ON -DVTK_USE_X=OFF
 ../../cmake-3.19.4/bin/cmake --build . --config Release --parallel 64
 
 # Install application.

offline_ray_tracer/source/main.cpp

@@ -9,7 +9,6 @@
 #include <vtkPolyDataMapper.h>
 #include <vtkRenderer.h>
 #include <vtkRenderWindow.h>
-#include <vtkRenderWindowInteractor.h>
 #include <vtkSmartPointer.h>
 #include <vtkUnstructuredGridVolumeRayCastMapper.h>
 #include <vtkVolume.h>