Separate PREFIX and DESTDIR to allow staged installs

According to this https://www.gnu.org/prep/standards/html_node/DESTDIR.html#DESTDIR and this http://www.freebsd.org/doc/en/books/porters-handbook/porting-prefix.html `PREFIX` and `DESTDIR` are related but not connected (like it was with `PREFIX?=$(DESTDIR)/usr/local`).