RF: don't use rpath for library install names

Instead of setting rpath to be ``@loader_path/../.dylibs`` (or similar)
then setting install_name to be ``@rpath/my_library.dylib``, make
install name be ``@loader_path/../dylibs/my_library.dylib``.  It's a
little bit less code, and it avoids an error I got running out of header
space writing the rpath for a scipy wheel.