[MacPorts] #56294: boost: update to 1.68.0

[MacPorts]

#56294: boost: update to 1.68.0
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  Reporter:  michaelld  |      Owner:  ryandesign
      Type:  update     |     Status:  assigned
  Priority:  Normal     |  Milestone:
 Component:  ports      |    Version:
Resolution:             |   Keywords:  haspatch maintainer
      Port:  boost      |
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Comment (by michaelld):

 CMake is aware of this change:
 https://gitlab.kitware.com/cmake/cmake/blob/master/Modules/FindBoost.cmake#L43
 . One just has to use the correct module name, which now depends on which
 version of Boost is installed ... so, some logic needs to be in place for
 projects looking for any Boost Python-based ABI (e.g., "python" or
 "numpy"). Fun fun!

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Ticket URL: <https://trac.macports.org/ticket/56294#comment:12>
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