Add support for working in conjuction with other buildpacks.

If you are using heroku-buildpack-multi and/or heroku-buildpack-apt with
heroku-buildpack-python, you probably want to be smart about prepending
your paths, and also accept the build environment's LD_LIBRARY_PATH and
LIBRARY_PATH.

The ${STUFF:+} alternative expansion used here is to avoid adding
extraneous : characters to these path variables.