[MacPorts] #60331: Document compiler.cxx_standard

[MacPorts]

#60331: Document compiler.cxx_standard
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  Reporter:  iay          |      Owner:  (none)
      Type:  enhancement  |     Status:  new
  Priority:  Normal       |  Milestone:
 Component:  guide        |    Version:
Resolution:               |   Keywords:
      Port:               |
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Comment (by iay):

 I added a PR for the guide: https://github.com/macports/macports-
 guide/pull/35

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Ticket URL: <https://trac.macports.org/ticket/60331#comment:5>
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