Fix configure checks in macOS conda-forge environments

[speth]

This is mainly an update to our workaround for SCons/scons#2127, where SCons ignores directories specified for the RPATH on macOS. The issue is that if we're getting libc++ from a non-standard location, like a Conda install, then that needs to be on the RPATH before we even start the "configure" process.

If applicable, fill in the issue number this pull request is fixing

Fixes #1628

Checklist

• [x] The pull request includes a clear description of this code change
• [x] Commit messages have short titles and reference relevant issues
• [x] Build passes (scons build & scons test) and unit tests address code coverage
• [x] Style & formatting of contributed code follows contributing guidelines
• [x] The pull request is ready for review

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 73.13%. Comparing base (795965e) to head (ea65925).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1768 +/- ## ======================================= Coverage 73.12% 73.13% ======================================= Files 381 381 Lines 54174 54174 Branches 9223 9223 ======================================= + Hits 39615 39618 +3 + Misses 11598 11596 -2 + Partials 2961 2960 -1 ```

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[ischoegl]

I can confirm that this works on a newly updated Sonoma 14.6.1 (M2).