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

[Christian Maeder]

Robert Dockins wrote:
> I've now built a Shellac-editline package using the bindings from the 
> editline-0.2.  It's on hackage[1].  I've run the package through a quick 
> sanity check, but I have not extensively tested it.

Thanks, I've compiled it and run a short test on an Intel Leopard with
success.

> [1]http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Shellac-editline-0.9

I wasn't amused to also change my sources to use editlineBackend instead
of readlineBackend, besides changing the import to
System.Console.Shell.Backend.Editline.

Maybe the interfaces of the packages could be identical and one gets a
warning when both packages are installed. (I suppose one package must be
 hidden then, during compilation.)

1. In order to compile under MacOS with editline and under Linux with
readline it would be nice to use the same sources.

In order to avoid several #ifdefs (maybe differently or suboptimal
given) in user sources (to achieve 1.) such #ifdefs (or wrappers)
should be part of the libraries (including the documentation for the
license differences)!

Cheers Christian