[MacPorts] #62805: et @6.1.7: builds failing for all macOS releases earlier than 10.15

[MacPorts]

#62805: et @6.1.7: builds failing for all macOS releases earlier than 10.15
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  Reporter:  mascguy  |      Owner:  l2dy
      Type:  defect   |     Status:  assigned
  Priority:  Normal   |  Milestone:
 Component:  ports    |    Version:
Resolution:           |   Keywords:
      Port:  et       |
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Comment (by l2dy):

 There are two types of error. This one is a "bug" in base that
 `compiler.cxx_standard 2017` is not enforced and `<filesystem>` is
 missing.

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Ticket URL: <https://trac.macports.org/ticket/62805#comment:2>
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