Change configury logic to prefer external hwloc/libevent/pmix

[jsquyres]

Based on Dallas March 2018 face-to-face meeting notes, we want to shift the focus in v4.0.x away from the embedded hwloc, libevent, and pmix to external versions of these libraries.

Specifically, change the configury logic:

1. If user preference for external was specified:
   • Look for external support.
   • If external support is found and is compatible (e.g., the version/API/ABI/whatever is good), use it.
   • Otherwise, abort configure
2. If user preference for internal was specified:
   • Use it.
3. If no user preference was specified, look for external support.
   1. If external support is found:
      • If the external version is compatible (e.g., the version/API/ABI/whatever is good):
        • If the external version is >= the internal version, use the external version.
        • If the external version is < the internal version, emit a warning and use the internal version.
      • If the external version is not compatible:
        • Emit a warning and use the internal version.
   2. If external support is not found
      • Use the internal version.

Also add relevant ompi_info output somewhere indicating whether we are using internal or external versions, and give the reason why.

[jsquyres]

The bullets above were initially created from notes from the Dallas March 2018 meeting. Discussion welcome, especially before I start implementing. 😄

[ggouaillardet]

What is the rationale for using the internal version if external version < internal version but the external version is compatible ?

At first glance, I would simply print a warning and use the external version.

[jsquyres]

@ggouaillardet The thought there was that if the user didn't specify a preference, then we should just give them the latest version, regardless of whether it is internal or external.

[jsquyres]

Per discussion on 2018-07-10 webex, for PMIx:

• If external PMIx < v1.2.5, it is not compatible
• External PMIx v1.2.5 is compatible, but is not desirable because there are PMIx v2.0.x features that the OMPI code base uses.

[jsquyres]

Note that the current status of master moved a few steps towards this scheme, but it also did a few things that are explicitly not in this scheme.

For example, if the external component is configured successfully, it doesn't even try with the internal component -- which is contradictory to #5031 (comment): "The thought there was that if the user didn't specify a preference, then we should just give them the latest version, regardless of whether it is internal or external."

So there is definitely still work to be done.

[ggouaillardet]

IIRC, when the end user does not explicitly request anything, the external component version is tested, and is not selected if its version is lower than the embedded component.

[gpaulsen]

Can we close this issue now that PR #5472 has been merged to v4.0.x ?