[gtkada] another path error when specified DESTDIR during installation

[Emmanuel Briot]

> But it isn't where it is going to be used.
>
> I would typically use it by setting "DESTDIR" to "/tmp/gtkada" and "prefix" to
> "/usr" when I build the library, but only to package the library for later use.
> When I actually use the library I would copy the package to "/usr" and not to
> "/tmp/gtkada/usr".

Ah, that makes more sense indeed, thanks for the additional details. I agree it 
makes more sense that way, then.
I will patch GtkAda

Emmanuel