[gtkada] another path error when specified DESTDIR during installation

[Emmanuel Briot]

> No.  That is what you use "prefix" for.  "DESTDIR" is for
> installing to a different directory than where the files are
> actually going to be used.

Yes, I know the difference between prefix and DESTDIR.
But in the case of an absolute path coded in a project, we definitely want both 
${DESTDIR}${prefix}, since that's the actual path where the library is available.