[CMake] add_custom_command depends on output from add_custom_target

[SK]

I know add_custom_target does not produce output as far as CMake is
concerned.  My situation is as follows:

A traditional make process that I cannot modify produces a target on
which I must do some post-processing.

I use add_custom_target to force the makefile to always run. I use
add_custom_command for my post-processing steps.

How do I setup a dependency for the post-processing steps? I don't
want to run these steps unless the external make actually updated its
target.

The add_custom_command cannot use the name of the add_custom_target as
a DEPENDS. I tried this and it assumes that the add_custom_target name
is just a file and cannot find it.

Thanks for any help!