Add RPATH to message libraries?

[nnmm]

To avoid the problem described in https://github.com/ros2-rust/ros2_rust/pull/180, one option (I think) would be to have each message crate add an RPATH entry pointing to the library.

This path might not always be valid, but if the library is not found at the specified path, it's not an error, but simply the same behavior that we have today.

Would this be a good solution?