Per https://github.com/open-mpi/ompi/issues/5031, if the user didn't specify a particular PMIx installation, then default back to the internal version if it is newer than the discovered external one. PMIx doesn't yet provide a full signature so we have to just get as close as possible for now.
Signed-off-by: Ralph Castain <rhc@open-mpi.org>
(cherry picked from commit 1e6aaf7f226f5a4d940e544079e3977229746c11)
