The PMIX_MODEX and PMIX_INFO_ARRAY macros were removed from the PMIx 3.1 standard.
Open MPI does not really need them (they are only used to be reported as not supported),
so smply #ifdef protect them to support an external PMIx v3.1
Refs. open-mpi/ompi#6247
Signed-off-by: Gilles Gouaillardet <gilles@rist.or.jp>