I've added dpatch to Build-Depends:

  http://hg.grml.org/fvwm-crystal-minimal/rev/9cb22910f60f

I'm waiting for a new upstream release though because the current one (not yet 
in the repository but just in hg yet - version 3.0.5-1) has some unaddressed 
issues left (I've informed upstream). Debian's fvwm-crystal version is outdated 
as well (3.0.4) so I've sent a bugreport against Debian's BTS. Hopefully 
someone maintains the fvwm-crystal package within Debian so we can base our 
work on it again.

BTW: I personally don't use fvwm-crystal any longer - I appreciate any help 
regarding packaging fvwm-crystal[-minimal] therefore.