stop using an absolute rpath

This is a bad default, because the binaries will point at an absolute
path regardless of where they are moved. This opens up a security issue
for packages, because they will attempt to load libraries from a path
that's often owned by a regular user.

Every Rust binary is currently flagged by Debian, Fedora and Arch lint
checkers as having dangerous rpaths. They don't meet the requirements to
be placed in the repositories without manually stripping this from each
binary.

The relative rpath is still enough to keep the binaries working until
they are moved relative to the crates they're linked against.

http://wiki.debian.org/RpathIssue
https://fedoraproject.org/wiki/Packaging:Guidelines#Beware_of_Rpath