[MacPorts] #64620: gcc10 @10.3.0_1+universal fails to build on 10.13

[MacPorts]

#64620: gcc10 @10.3.0_1+universal fails to build on 10.13
------------------------+----------------------
  Reporter:  JohnFHall  |      Owner:  (none)
      Type:  defect     |     Status:  assigned
  Priority:  Normal     |  Milestone:
 Component:  ports      |    Version:  2.7.1
Resolution:             |   Keywords:
      Port:  gcc10      |
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Comment (by kencu):

 that header should only be called for if you’re building with a c++17
 capable compiler that has c++17 mode enabled.

 https://github.com/gcc-
 mirror/gcc/blob/d25a93742020c2f1efa72d9908406bef6936ab38/libstdc%2B%2B-v3/include/bits/basic_string.h#L47

 In which case, the header should be there.

 Which part of this is broken?

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Ticket URL: <https://trac.macports.org/ticket/64620#comment:2>
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