Open MPI currently fails to build with scheduler=slurm if +pmix is
not given with a fatal error due to ``config_args +=
self.with_or_without('pmix', ...)`` resulting in --without-pmix.
However, Open MPI's configure points out "Note that Open MPI does
not support --without-pmix."
The PR only adds "--with-pmix=PATH" if +pmix is part of the spec.
Otherwise, nothing is added and Open MPI can fall back to its
internal PMIX sources.
(The other alternative would be to depend on +pmix in for
scheduler=slurm as is done for +pmi.)
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