Try to fix the graphics loading when opengl is not found

CMakeLists.txt

@@ -640,12 +640,12 @@ if(HAVE_FLTK OR HAVE_QT OR ENABLE_GRAPHICS)
   endif(ENABLE_OSMESA)
 
   find_package(OpenGL REQUIRED)
-  if(OPENGL_GLU_FOUND)
+  if(OPENGL_GLU_FOUND AND OPENGL_FOUND)
     add_subdirectory(Graphics)
     set_config_option(HAVE_OPENGL "OpenGL")
-  else(OPENGL_GLU_FOUND)
+  else(OPENGL_GLU_FOUND AND  OPENGL_FOUND)
     message(SEND_ERROR "Could not find GLU: disabling OpenGL support")
-  endif(OPENGL_GLU_FOUND)
+  endif(OPENGL_GLU_FOUND AND  OPENGL_FOUND)
 endif(HAVE_FLTK OR HAVE_QT OR ENABLE_GRAPHICS)
 
 if(HAVE_MESH OR HAVE_PLUGINS)