Add LD_LIBRARY_PATH as default

Apparently, there's no way to set environment variables from the buildpack (at least not after some 30 min of looking around for a solution). This makes sense IMO since slug compilation takes part on different dynos. There seems to be a labs feature allowing this, but it's not encouraged and it doesn't seem to be any better than running an extra heroku command, so I just added this in the docs.