[MacPorts] #56041: Update compiler selection for 10.6-10.8 with libc++

[MacPorts]

#56041: Update compiler selection for 10.6-10.8 with libc++
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  Reporter:  jmroot       |      Owner:
      Type:  enhancement  |     Status:  new
  Priority:  Normal       |  Milestone:  MacPorts 2.5.0
 Component:  base         |    Version:  2.4.2
Resolution:               |   Keywords:
      Port:               |
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Comment (by ryandesign):

 Possibly related to #55444 and/or #56042.

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Ticket URL: <https://trac.macports.org/ticket/56041#comment:1>
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