[MacPorts] #65532: isl @0.25: builds failing for 10.7 and 10.13; needs fixes for c++17 detection

[MacPorts]

#65532: isl @0.25: builds failing for 10.7 and 10.13; needs fixes for c++17
detection
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  Reporter:  dyne2meter  |      Owner:  MarcusCalhoun-Lopez
      Type:  defect      |     Status:  assigned
  Priority:  Normal      |  Milestone:
 Component:  ports       |    Version:  2.7.2
Resolution:              |   Keywords:
      Port:  isl         |
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Comment (by kencu):

 on 10.6, we install a much newer libc++, so perhaps paradoxically, it is a
 much more capable system in some ways than “newer” systems

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Ticket URL: <https://trac.macports.org/ticket/65532#comment:15>
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