Disable xdr, unless HAVE_RPC_RPC_H is set

This fixes #33 (closed) for me.

Note: A CMake test for rpc.h is still missing, but unless used by dune-grid I see no reason to add it.

assigned to @ansgar

It's only used by UG's mgio.cc to in-/output UG's grid structore to a file. We don't currently use this, but the functionality shouldn't be removed though as it should be helpful for eventually implementing checkpoint/restore support. So just hiding it behind an #ifdef 0 is fine for now.

merged

mentioned in commit d3af23d9

mentioned in commit 5ae4b75c

mentioned in merge request !83 (merged)