[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:
      Type:  enhancement  |     Status:  new
  Priority:  Normal       |  Milestone:
 Component:  base         |    Version:
Resolution:               |   Keywords:
      Port:               |
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Comment (by ryandesign):

 We need to be careful that, in simulating another C++ library, we only
 change the default C++ library, and do not disturb ports that override
 that default. For example:

 * mongodb always uses libc++, even on systems that default to libstdc++
 * giza-pp always uses libstdc++, even on systems that default to libc++

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Ticket URL: <https://trac.macports.org/ticket/55471#comment:1>
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