Makefile changes to allow changing install dir

Please merge this:

commit 77151a25556b555c752495ac66c2460fae7f11eb
Author: Vitaly Greck <vintozver@ya.ru>
Date:   Wed Dec 19 13:43:41 2012 +0300

    Allow Makefile accept env variables (for compatibility with rpm and deb package builders)
    
    DESTDIR is the root by default
    PREFIX is /usr/local by default (may be different or some OSes)