[MacPorts] #59780: cmake fails to build on Leopard 10.5: error: 'shared_mutex' is unavailable: introduced in macOS 10.12

[MacPorts]

#59780: cmake fails to build on Leopard 10.5: error: 'shared_mutex' is unavailable:
introduced in macOS 10.12
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  Reporter:  rmottola  |      Owner:  michaelld
      Type:  defect    |     Status:  assigned
  Priority:  Normal    |  Milestone:
 Component:  ports     |    Version:
Resolution:            |   Keywords:  leopard
      Port:  cmake     |
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Comment (by rmottola):

 seems gcc7 works? strange!

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Ticket URL: <https://trac.macports.org/ticket/59780#comment:10>
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