Update PMI detection logic

Update the PMI detection logic to support oddball case where both lib and lib64 exist, sometimes seen in Slurm installations. Also, better support Cray installations of all ages

Master commits:

https://github.com/open-mpi/ompi/commit/169a1866b8b3035cd118367077bd9ff620dc7e1e
https://github.com/open-mpi/ompi/commit/b1a58726ac17e596f25d732c97349d6156a9fdd1

Shouldn't we AC_CHECK_HEADER to ensure that pmi.h is valid, too?