Replace XDR code with BSD-relicensed copy

The bottom part of src/xdrfile/xdrfile.c contains Sun RPC code, which is licensed under a non-free license. Several copies of that code were relicensed by Oracle to a BSD license.

Please replace your version with a BSD-licensed one. I have filed a solution for this at mdtraj's related bug: mdtraj/mdtraj#2204

Thanks for the heads up. I'm out today but should be able to get to it tomorrow.