Split INSTALLPATH into DESTDIR+PREFIX

PREFIX is the final path where the app will be installed
DESTDIR is the intermediate destination used by package managers

Needed for example to make the program installable by homebrew. Here's a working formula: https://gist.github.com/zimbatm/b2edd419d8eb4b375077

Update: the default for PREFIX should be /usr/local but I didn't want to change the semantic.