[MacPorts] #57341: opencv @3.4.1_2 +contrib error - build failure

[MacPorts]

#57341: opencv @3.4.1_2 +contrib error - build failure
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  Reporter:  lbarrows    |      Owner:  stromnov
      Type:  defect      |     Status:  assigned
  Priority:  Normal      |  Milestone:
 Component:  ports       |    Version:  2.5.4
Resolution:              |   Keywords:  OpenCV contriub protobuf build
      Port:  opencv      |  breaks broken High Sierra
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Comment (by kencu):

 probably the same error as the caffe build, which doesn't build against
 protobuf3 due to needing c++11.

 One quick & easy test is to try building it with `clang-7.0`; if that
 fixes it, then we know it's a c++11 thing.

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Ticket URL: <https://trac.macports.org/ticket/57341#comment:1>
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