[MacPorts] #55471: Use separate portindexes for libc++ on older systems

[MacPorts]

#55471: Use separate portindexes for libc++ on older systems
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  Reporter:  ryandesign   |      Owner:  neverpanic
      Type:  enhancement  |     Status:  assigned
  Priority:  Normal       |  Milestone:  MacPorts 2.5.0
 Component:  base         |    Version:
Resolution:               |   Keywords:
      Port:               |
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Comment (by mojca):

 Thanks a lot for the explanation. I didn't catch all the details of the
 discussion during the meeting (and didn't want to ask too much as it was
 already taking way too much time about this particular ticket).

 I'll respond to other questions later, but feel free to proceed with your
 PR.

 The issue won't be solved for users of `libc++` on legacy systems, but
 it's the first step in the right direction and we might indeed want to
 coordinate a few other steps. (If you always append `cxx_stdlib`, you'll
 need a change everywhere, even for 10.13.)

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Ticket URL: <https://trac.macports.org/ticket/55471#comment:27>
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