[gtkada] DESTDIR blocked, patch provided

[Emmanuel Briot]

> Ah, maybe DESTDIR is a very special case that's handled differently that 9.5 states.

It works on OSX and Linux, though.

> I certainly appreciate that!
> I'm trying to get 2.24.4 into FreeBSD ports now and trying to figure out why DESTDIR malfunctioned took a while.

But FreeBSD does not use GNU make, I believe… which is likely where the different behavior comes from.
As a general rule, AdaCore tools are meant to be compiled with GNU make, even if we certainly try to
accommodate other cases when possible.

Emmanuel