LP 799718: Pass the DESTDIR variable to setup.py

LP 799718: Pass the DESTDIR variable to setup.py

This allows python modules to be built when creating packages.

Per comment from Dan Scott on the LP bug:

http://pubs.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap03.html#tag_03_266
says "Multiple successive slashes are considered to be the same as one
slash." so in theory "--root=$(DESTDIR)/" should be just as safe as
"--root=$(DESTDIR)///", but why not go the distance and use 3 slashes to
avoid any possibility of some oddball POSIX implementation that treats 2
slashes differently for some reason?

Signed-off-by: Ben Webb <bjwebb67@googlemail.com>
Signed-off-by: Dan Scott <dscott@laurentian.ca>