[gtkada] Patch: Repair the support for directory variables.

[Nicolas Setton]

Hello Björn,

> To upgrade the GTKada package in Fedora I had to fix the support for
> directory variables again. This version of the patch applies to the
> trunk.

Thank you for the patch! If you don't mind, could you please submit this 
as a github pull request for

   https://github.com/AdaCore/gtkada

(We would like to make github pull requests the official way of 
contributing patches)

And same question for your testgtk patch.

Nicolas