>> ISTM that it may be more helful to do:
>>
>>   ifndef JADE
>>   #error "no jade found on your system, cannot generate the documention"
>>   endif
>
> We could use $(missing) for that, which is already used for bison, flex,
> and perl.
I'm fine with "$(missing)" or whatever else that provides a clear message.
Oops, not "#error", but "$(error  ...)", I was mixing cpp & make above...
However the example in the doc Makefile for "collageindex.pl" is on the 
heavy side, as it suggests that every use of such commands should be put 
in ifdef/else/endif. ISTM that a dependence-based solution would be 
simpler.
-- 
Fabien.