[gtkada] improvements to the GtkAda build system

[Arnaud Charlet]

> Then I'll proceed to the DESTDIR support. Unlike the patches that John
> Marino
> submitted, this one doesn't delete the rm commands. Instead it applies
> DESTDIR
> even to those, so that when installation is done into a staging directory,
> nothing will be deleted outside of the staging directory. The patch doesn't
> delete any chmod commands either.

Looks good, patch committed.

Arno