[Cmake] recursive custom targets

[Stefan Schmidt]

Sorry, I've yet another question on which I couldn't find any info:

How can I create a custom target that is made recursively?

My documentation needs to be generated in a 2-pass process, so I'd like to 
have a "make docprep" and a "make doc" target, which both are done 
recursively for the whole project.

Is this possible?

Thanks in advance,
Stefan