[MacPorts] #44129: cmake: fix build against libc++ on Lion

[MacPorts]

#44129: cmake: fix build against libc++ on Lion
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  Reporter:  mojca@…  |      Owner:  css@…
      Type:  defect   |     Status:  new
  Priority:  Normal   |  Milestone:
 Component:  ports    |    Version:
Resolution:           |   Keywords:  lion
      Port:  cmake    |
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Comment (by jeremyhu@…):

 That's a good idea.  We can conditionalize the fallback list based on
 cxx_stdlib.

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Ticket URL: <https://trac.macports.org/ticket/44129#comment:9>
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