cmake_minimum_required(VERSION 3.4)

add_custom_command(OUTPUT ${CMAKE_BINARY_DIR}/myout
    COMMAND ${CMAKE_COMMAND} -E touch ${CMAKE_BINARY_DIR}/myou
    COMMAND ${CMAKE_COMMAND} -E echo "running custom command!"
    DEPENDS ${CMAKE_SOURCE_DIR}/some_file
)

add_custom_target(myout-target ALL DEPENDS ${CMAKE_BINARY_DIR}/myout)