Simplify PMI configure logic. Master commit:

open-mpi/ompi@75d8a7f

@jsquyres please review
FWIW: when doing a simple assignment of one shell variable to another, you do not need quotes.  I.e.:

```shell
save_CPPFLAGS="$CPPFLAGS"
# is functionally equivalent to (but slightly less efficient than)
save_CPPFLAGS=$CPPFLAGS
```

Unexpectedly, this is true even if the RHS contains (or may contain) whitespace.  Using quotes is slightly less efficient because it basically makes a new string and copies the value into it, and then does the assignment.

In the grand scheme of things, this minor inefficiency is irrelevant.  This is just a random point of knowledge for ya in the future.

Not asking you to change this for this PR; just a random point of knowledge for ya in the future...