build OpenMPI examples with XPMEM support

[reidpr]

Build the OpenMPI examples with XPMEM support so that they can use that kernel module for single-copy same-node communication, because it performs better than cross-memory attach (CMA) and may avoid CMA's access control issues (#128).

However, the kernel module must be secure first. See hpc/xpmem#25.

[loveshack]

I'm not sure of relative merits otherwise, but perhaps you could use knem. Goglin says it's safe: https://lists.gforge.inria.fr/pipermail/knem-devel/2019-September/000183.html