Update slurm pmi configury to account for pmix

When Slurm is built against PMIx, some installations place a copy of the
PMIx library that Slurm is linking against in the Slurm PMI location.
Current configury ignores that location. The desired behavior is to look
for a PMIx lib in that location when --with-pmi is given. If the user
also specifies --with-pmix and gives a different location, then override
anything previously found and look for it where the user directed.

Signed-off-by: Ralph Castain <rhc@pmix.org>

Author: rhc54