fixed warnings in library_rpath.cmake
fixed warnings in library_rpath.cmake

Only recurse if the current library is a TARGET, not when it is just
non-empty.
Also remove duplicates from the collected rpath.

agra-uni-bremen/metaSMT,hoangmle/metaSMT,finnhaedicke/metaSMT,nbruns1/metaSMT,hoangmle/metaSMT,nbruns1/metaSMT,finnhaedicke/metaSMT,agra-uni-bremen/metaSMT,agra-uni-bremen/metaSMT,finnhaedicke/metaSMT,hoangmle/metaSMT,nbruns1/metaSMT