Clarify in README what --with-hwloc does in its different use cases.
Also, ensure that the behavior when specifying `--with-hwloc` is the
same as if that option is not specified at all. This is what we did
in Open MPI <= v3.x; looks like we inadvertantly caused `--with-hwloc`
to be synonymous with `--with-hwloc=external` in v4.0.0.
Signed-off-by: Jeff Squyres <jsquyres@cisco.com>
(cherry picked from commit 18c3e1af5ef281e8b502a7ad778256889cc707d1)
814f3e9caa