setup.py: RPath Issue on OSX with ADIOS1?

@ChristopherMayes observed an rpath issue with `setu.py` based installs from PyPI with `0.7.1a` regarding RPaths of our ADIOS1 wrapper library.

In 0.8.0 we will only build this when ADIOS1 is actually used, but the problem could still be there (when ADIOS1 is present):

```
>>> import openpmd_api
Traceback (most recent call last):
File "", line 1, in
ImportError: dlopen(/opt/local/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/openpmd_api.cpython-36m-darwin.so, 2): Library not loaded: @rpath/libopenPMD.ADIOS1.Serial.dylib
Referenced from: /opt/local/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/openpmd_api.cpython-36m-darwin.so
Reason: image not found
```

### Environment

HDF5: 1.10.4
h5py: 2.9.0
macOS Mojave (latest)
python 3.6.8