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
Merge pull request #80 from zimbatm/master

Split INSTALLPATH into DESTDIR+PREFIX

Thanks, merged.

Rmlint's developement version uses SCons by the way.
If Im not recalling that wrong, this should be supported there already.

(https://github.com/sahib/rmlint/tree/develop)