MACRO(USE_VRPN)
  IF ((NOT VRPN_FOUND) AND (${ARGC} LESS 1))
    USING_MESSAGE("Skipping because of missing VRPN")
    RETURN()
  ENDIF((NOT VRPN_FOUND) AND (${ARGC} LESS 1))
  IF(NOT VRPN_USED AND VRPN_FOUND)
    SET(VRPN_USED TRUE)
    add_definitions(-DHAVE_VRPN)
    INCLUDE_DIRECTORIES(SYSTEM ${VRPN_INCLUDE_DIRS})
    SET(EXTRA_LIBS ${EXTRA_LIBS} ${VRPN_LIBRARIES})
  ENDIF()
ENDMACRO(USE_VRPN)