cabal: question on compiling build prerequisites

[Henning Thielemann]

On Mon, 9 Dec 2013, Sven Panne wrote:

> I thought about a similar scenario recently, and I would like to hear
> if there is a clean cabal solution for this, too. Ripping out a helper
> program which has only a single use case (support for the "real"
> package) and which totally has to stay in sync with that "real"
> package would not be very nice. It would just be an artifact of a
> (missing?) cabal feature.

The question is, how far should this feature reach? What if the 
preprocessor needs a preprocessor itself? E.g. a preprocessor might be 
built using a parser generator, then you need this parser generator as 
build-tool for the preprocessor build-tool.