crop marks in PDF for printing

[Federico Bruni]

Hi folks

I have a urgent request.
Tomorrow morning I need to print a book and I've just been asked to add 
"crop marks" (I think this is the right expression) to the final PDF. 
IIUC crop marks are not needed when printing with normal printers, but 
it's needed for serious digital and offset printing machines.

I wonder how can I get this in LilyPond.
I didn't find anything in the documentation or the user list, except 
this question which got no reply:
https://lists.gnu.org/archive/html/lilypond-user/2009-07/msg00319.html

I guess that I'll have to use some external applications.
Can anybody give me a hint?

Many thanks
Federico