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

[MacPorts]

#59780: cmake fails to build on Lion 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:
      Port:  cmake     |
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Comment (by kencu):

 by the way, I think I've mentioned this to you before, but I do keep a
 reasonably up-to-date overlay repos for older systems that pegs (or fixes)
 these ports when I find them broken and unpegs them when we/I get around
 to fixing them in MacPorts. There's also a nice version of libsdl2 that
 works on 10.4 and 10.5 there.

 https://github.com/kencu/TigerPorts\\
 https://github.com/kencu/LeopardPorts\\
 https://github.com/kencu/SnowLeopardPorts\\
 https://github.com/kencu/LionPorts

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Ticket URL: <https://trac.macports.org/ticket/59780#comment:5>
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