Switch to from chrpath to patchelf on Linux (just like #11)

This is a do-over of #11 since @peter1000 deleted his original fork.

The main reason I'm doing this is that I can't successfully build a Perl Linux package without this switch. (See conda/conda-recipes#91).

`patchelf` doesn't suffer from the same limitation as `chrpath` that the `rpath` length can't increase when you're changing it. 

@ilanschnell, there is a `patchelf` recipes in the conda-recipes repository already, which should probably be added to Anaconda if this PR gets merged.