[gtkada] another path error when specified DESTDIR during installation

[Emmanuel Briot]

> Found another install bug, when specific DESTDIR in make install, the
> installed gtkada.gpr, etc, files (generated from
> projects/generate_prj.sh) contain a path that include the DESTDIR
> path.

I do not understand this one though.

libdir is set in src/Makefile.common to a value that does not contain DESTDIR.
So we need to add it in the calls to generate_prj.sh.

We do want the generated projects to include DESTDIR I believe, otherwise the 
linker will not find the libraries.

Emmanuel