[MacPorts] #59113: gcc8: cannot find systems headers with Xcode 11

[MacPorts]

#59113: gcc8: cannot find systems headers with Xcode 11
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  Reporter:  abyrvalk  |      Owner:  (none)
      Type:  defect    |     Status:  new
  Priority:  Normal    |  Milestone:
 Component:  ports     |    Version:
Resolution:            |   Keywords:
      Port:  gcc8      |
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Comment (by jmroot):

 Restoring the 10.14 SDK (whether by downgrading Xcode or otherwise), or
 adding {{{-isysroot`xcrun --show-sdk-path`}}} to your compiler flags seem
 to be the currently known workarounds.

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Ticket URL: <https://trac.macports.org/ticket/59113#comment:3>
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