[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:
      Port:  boost      |
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Comment (by michaelld):

 Given the ~4 months for projects to fix their issues with Boost 1.67.0,
 and possible more issues now with 1.68.0, let's test this release with
 ports that require Boost to make sure the vast majority work. I would like
 to get this updated Boost in place sooner rather than later, assuming
 compatibility is good.

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Ticket URL: <https://trac.macports.org/ticket/56294#comment:5>
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