osx/universal: make the library name relocatable

If a consumer links against a library, it references that library by the id attribute in that library. If the id attribute contains a absolute path, then it will be used as the full path to link against breaking relocatable builds entirely.

Fix by always making the id of the generated library be of the form @rpath/libname.dylib. This also requires that users of the library set up an appropriate rpath entry.

Fixes #362 (closed)