[Cmake] Question: how to use ADD_CUSTOM_COMMAND?

[Shunming Fang]

Hello, Any,
When I implement ADD_CUSTOM_COMMAND, I found it seems need SOURCE to
trigger my command to work, But my original idea is to build doc/html
for my package in one subdir without any source code, So the question is
if no source option  was used, How I can implement ADD_CUSTOM_COMMAND ? or
in other word, How I can trigger this command work?
Thanks
Fang