darcs patch: [PROOF OF CONCEPT] build readline package with libedit

[Christian Maeder]

Robert Dockins wrote:
> If you want a package that multiplexes the editline package or the readline 
> package depending on avaliability, that would be easy to do.  It would just 
> require wraping up the #ifdef stuff needed into a single place.  I can 
> probably find a few minutes this afternoon/evening to do this.

Yes, that's exactly what I want, although a third module name is needed.
How do you set up the cabal file for such a package?

The license entry could simply mention the possible readline dependency.

Cheers Christian

(Maybe that's useful for other packages, too, that can be realized via
readline or editline, or even for the common bits of the readline and
editline packages, since it's not the module
System.Console.Editline.Readline ifself)