[CMake] big custom command dependency problem

[Juan Sanchez]

I created a custom command to generate a cpp file.

I then use this cpp file to create a library as a separate add_library
command.

Whenever I type make, this cpp file is regenerated, and the library is
recreated.  Compiling this file is super expensive.  Nothing has changed
between invocations of make.

I stated the dependencies in the custom command.  Why is it always
recreating the cpp file?

Is there anyway to stop this behavior?

Turning off dependency checking is not an option since it is needed for
everything else.  This is gmake on a linux box.

Thanks,

Juan