Makefile: DESTDIR is not added to PREFIX if PREFIX is manually given

[nanonyme]

When used with packaging, we might want prefix to point to /usr and collect package artifacts into /staging-dir. In this case we would set PREFIX=/usr and DESTDIR=/staging-dir. This results in make actually installing files under /usr (which might not be mutable) instead of under /staging-dir/usr.

[kolyshkin]

Thanks for reporting, proposed fix is #2578