Check for the presence of setenv. If it is present, then use it in opal_setenv when setting values in the environ

Port of open-mpi/ompi@0726388

@jsquyres please review
Pull over two cleanups from the master:

open-mpi/ompi@2c9987b7d18ea6b50ad8233d8d99565bc6fd644e
open-mpi/ompi@4e4920a0fdb36e4baa3e2322f1664d75a58cab79
Merge pull request #76 from rhc54/cmr/check-setenv

Check for the presence of setenv. If it is present, then use it in opal_...