Tests fail on OS X

On OS X (10.11.3) the tests fail because a relative rpath is not allowed:

```
running...
Traceback (most recent call last):
File "crossmod_exception.py", line 7, in
import crossmod_exception_a
ImportError: dlopen(/Users/spkersten/Development/boost-github/boost/bin.v2/libs/python/test/crossmod_exception.test/darwin-4.2.1/debug/crossmod_exception_a.so, 2): Library not loaded: libboost_python.dylib
Referenced from: /Users/spkersten/Development/boost-github/boost/bin.v2/libs/python/test/crossmod_exception.test/darwin-4.2.1/debug/crossmod_exception_a.so
Reason: unsafe use of relative rpath libboost_python.dylib in /Users/spkersten/Development/boost-github/boost/bin.v2/libs/python/test/crossmod_exception.test/darwin-4.2.1/debug/crossmod_exception_a.so with restricted binary
```

I'm running the tests by executing bjam in boost/libs/python/test.