[MacPorts] #64345: qt6-qtbase fails to build on 10.14

[MacPorts]

#64345: qt6-qtbase fails to build on 10.14
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  Reporter:  jmroot      |      Owner:  (none)
      Type:  defect      |     Status:  new
  Priority:  Normal      |  Milestone:
 Component:  ports       |    Version:
Resolution:              |   Keywords:  mojave
      Port:  qt6-qtbase  |
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Comment (by chrstphrchvz):

 The relevant code is guarded by `if (@available(macOS 10.15, *)) {…}`:
 [https://github.com/qt/qtbase/blob/v6.2.1/src/gui/platform/darwin/qapplekeymapper.mm#L525].
 I’m not too familiar with this Objective-C feature, but why does it not
 prevent the errors? Could it be that the Xcode clang on the builder or SDK
 are too old, or would it complain during configure that were the case?

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Ticket URL: <https://trac.macports.org/ticket/64345#comment:2>
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