MACRO(USE_CG)
  IF ((NOT CG_FOUND) AND (${ARGC} LESS 1))
    USING_MESSAGE("Skipping because of missing CG")
    RETURN()
  ENDIF((NOT CG_FOUND) AND (${ARGC} LESS 1))
  IF(NOT CG_USED AND CG_FOUND)
    SET(CG_USED TRUE)
    USE_PTHREADS()
    USE_OPENGL()
    ADD_DEFINITIONS(-DHAVE_CG)
    INCLUDE_DIRECTORIES(SYSTEM ${CG_INCLUDE_DIRS})
    SET(EXTRA_LIBS ${EXTRA_LIBS} ${CG_LIBRARIES})
  ENDIF()
ENDMACRO(USE_CG)