use data-files without installation

[Henning Thielemann]

On Fri, 3 Mar 2017, Edward Z. Yang wrote:

> One of the tracking bugs we have for this issue:
> https://github.com/haskell/cabal/issues/4120
> Note that the data file directory can be overridden
> with environment variables, that might be good enough to
> work around.
>
> It would be really nice if inplace data files worked.
> It is a little tricky to do, since the installed layout
> may not match the inplace layout, but maybe it would be OK
> if this were just opt in.

Thank you for the pointer to the ticket. I didn't find it myself because I 
did not expect that the problem has to do with Nix. :-)

I see that the problem is hard. Maybe paths relative to the executable 
could be another option?