cmake add_custom_command 无效

这样不行：

cmake_minimum_required(VERSION 2.8)
project(test)

add_custom_command(
    OUTPUT aaa
    COMMAND ls)

add_custom_target(target_name 
    DEPENDS aaa
)
      

cmake_minimum_required(VERSION 2.8)
project(test)

add_custom_target(target_name ALL
COMMAND ls)