[m-rev.] for review: bugfix for io.set_environment_var/4

[Peter Wang]

On Mon, 18 Apr 2016 09:49:23 +1000 (AEST), Julien Fischer <jfischer at opturion.com> wrote:
> 
> W.r.t to Mark's change: I think we also need to add
> 
>      :- pred have_set_environment_var is semidet.
> 
> so that programs can test whether the unerlying platfrom supports
> modifying the environment.
> 

Or just add a version of io.set_environment_var that returns a success
code instead of throwing an exception.

Peter