Add 'generic' support for PMI2 (previously, we checked for PMI2 only on Cray systems.)  If your resource manager (e.g. SLURM) has support for PMI2, then the --with-pmi configure flag will enable its usage. If you don't have PMI2, then you will fallback to regular old PMI1. This patch was submitted by Ralph Castain and reviewed and pushed by Josh Ladd. This should be added to cmr:v1.7:reviewer=jladd