COMP: Use vtkOpenGL.cmake for vtkRenderingExternal module

Rendering/External/CMakeLists.txt

@@ -15,6 +15,8 @@ set(${vtk-module}_HDRS
   )
 
 vtk_module_library(vtkRenderingExternal ${Module_SRCS})
+include(vtkOpenGL)
+vtk_opengl_link(${vtk-module})
 
 if ("${VTK_RENDERING_BACKEND}" STREQUAL "OpenGL2")
   set_target_properties(

Rendering/External/Testing/Cxx/CMakeLists.txt

@@ -11,3 +11,5 @@ VTK_ADD_TEST_CXX(${vtk-module}CxxTests tests
 VTK_TEST_CXX_EXECUTABLE(${vtk-module}CxxTests tests)
 TARGET_LINK_LIBRARIES(${vtk-module}CxxTests LINK_PRIVATE
   vtkRenderingExternal ${OPENGL_LIBRARIES} ${GLUT_LIBRARY})
+include(vtkOpenGL)
+vtk_opengl_link(${vtk-module}CxxTests)