Add option to build either w/ RPATH or RUNPATH

[hegner]

As discussed in #639 this PR adds an option whether to use RPATH or RUNPATH.
Please note that this PR sits on top of PR #908, so that would need to be accepted first.

[alalazo]

@hegner This is almost one year and a half old, and it has a number of conflicts I have never seen in any other PR 😄 Are you still maintaining this?

@tgamblin @scheibelp @davydden @adamjstewart Is this even a feature we want?

[adamjstewart]

I think this would be a good feature to have. I've talked to people who loved the idea of Spack, but the use of RPATH was a dealbreaker for them.

[tgamblin]

@alalazo @adamjstewart: I've been remiss in not getting this merged. It's an easy enough feature to add -- I want to get this in before SC.

[alalazo]

@tgamblin ping

[becker33]

This is superseded by #9168. Closing this as out-of-date, I think it will be easier to work from #9168.