[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):

 Rainer, can you please suggest how exactly you would solve this in what
 you think is ideal case? I would like to avoid spending another year or
 two with zero activity just because we cannot even decide what to do,
 while the patch would in fact be super straightforward.

 We can of course always add the `cxx_stdlib` to the name, but then you
 also need to change `selfupdate` (or whatever all those places are) to
 fetch a different `PortIndex` file from the server.

 In any case, can you please merge one of the two PRs and then we proceed
 with discussion about the changes in remaining two files.

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Ticket URL: <https://trac.macports.org/ticket/55471#comment:29>
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