[MacPorts] #62677: Add C++20 support for configure.cxx_standard

[MacPorts]

#62677: Add C++20 support for configure.cxx_standard
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  Reporter:  ryandesign   |      Owner:  (none)
      Type:  enhancement  |     Status:  new
  Priority:  Normal       |  Milestone:
 Component:  base         |    Version:
Resolution:               |   Keywords:
      Port:               |
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Comment (by jmroot):

 - https://clang.llvm.org/cxx_status.html#cxx20
 - https://gcc.gnu.org/projects/cxx-status.html#cxx20

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Ticket URL: <https://trac.macports.org/ticket/62677#comment:1>
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