I have not tested your patch but DESTDIR is simpler than you think, perhaps. You should do everything as specified by PREFIX and then, at install time, copy to $DESTDIR/$PREFIX if DESTDIR is defined, otherwise everything goes as before, ie, copy to $PREFIX.DESTDIR is useful because you can isolate the files to be installed from other files in the file system.
This is exactly what I did in the patch.
env['DEST_DIR'] = PARAMETERS.get('dest_dir', env['PREFIX'])
and some other minor adjustment.
Bo
