find system xdr properly
We should do a better job of finding headers needed by our XDR support (which e.g. could be provided by a library like libtirpc on e.g. Arch Linux). Currently we require that
#include <rpc/rpc.h> works, but could improve by using a FindXDR.cmake like https://github.com/Unidata/netcdf-c/blob/master/cmake/modules/FindXDR.cmake rather than failing to find
/usr/include/tirpc/rpc/rpc.h and thus falling back on our bundled XDR support. We always need the latter for Windows support (IIRC), but probably should avoid using it on *nix.