How do I add OpenGL to a project using CMake? -

i followed documentation (https://cmake.org/cmake/help/v3.0/module/findopengl.html) , wrote code below in cmakelists.txt...

if(not opengl_found)     message("there no opengl!") endif() 

...to try figure out how add opengl project using cmake. message keeps popping means system doesn't have opengl. how add system?