message: cmake(highgui): don't allow multiple builtin backends
diff: @@ -141,7 +141,7 @@ endif()
  if(TARGET ocv.3rdparty.win32ui)
  if("win32ui" IN_LIST HIGHGUI_PLUGIN_LIST OR HIGHGUI_PLUGIN_LIST STREQUAL "all")
  ocv_create_builtin_highgui_plugin(opencv_highgui_win32 ocv.3rdparty.win32ui "window_w32.cpp")
  - else()
  + elseif(NOT OPENCV_HIGHGUI_BUILTIN_BACKEND)
  set(OPENCV_HIGHGUI_BUILTIN_BACKEND "WIN32UI")
  list(APPEND highgui_srcs ${CMAKE_CURRENT_LIST_DIR}/src/window_w32.cpp)
  list(APPEND tgts ocv.3rdparty.win32ui)
  