Fixed linking of IECoreNuke for Nuke 8.

The libDDImage in Nuke 8 links against libRIPFramework, so that library must be available at build time in order for the configure tests to pass. The -L flag isn't used when searching for the dependencies of a library, so we use the -rpath-link flag to specify where libRIPFramework is to be found.

Unlike the -rpath flag, -rpath-link only has effect at compile time, and doesn't affect linking at runtime.

This pull request is an alternative to Lucio's #251.