fixes RPATH for dynamically linked shared library so files on osx

In some cases, dynamically linked shared library .so files are produced on OS X (PyQt5 for example). `install_name_tool -id` needs to be called to remove the build path from the `RPATH`, but `post:mk_relative_osx` only makes that call for .dylib files. This pull request also makes the call for .so files that `file` reports to be dynamically linked shared libraries.