[MacPorts] #56294: boost: update to 1.68.0

[MacPorts]

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

 * Attachment "boost_1.66.0_to_1.68.0.diff" added.


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