BundleUtilities: Fix bundle verification on Unix by considering rpaths.

This patch makes sure that the rpaths of the main executable are passed on during validation of a bundle. By default the old code added the exepath to ldpath, which would break if one has a standard linux file system structure which would split app & libs into /bin & /lib.

Please tell me what you think, I'll happily make improvements. That said, I am not exactly sure on how to test that stuff (this is more or less my first cmake contribution)