message: Make -DBUILD_BACKEND obsolete
diff: @@ -32,7 +32,7 @@ endforeach()
  project(mavsdk_superbuild)
  if (BUILD_BACKEND)
  - set(BUILD_MAVSDK_SERVER, ON)
  + message(FATAL_ERROR "The argument BUILD_BACKEND has been replaced by BUILD_MAVSDK_SERVER. To build mavsdk_server, use -DBUILD_MAVSDK_SERVER=ON.")
  endif()
  option(SUPERBUILD "Build dependencies" ON)
  