[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):

 One major ABI naming change: the Boost::Python library is renamed to
 include the major.minor Python version but with no "." ... so, "27" for
 Python 2.7 and "36" for Python 3.6 (for 2.,7 was "libboost_python-
 mt.dylib" is now "libboost_python27-mt.dylib"). Not sure if this is a
 change from 1.67.0 or 1.66.0 ... but it's important since any project
 using CMake to find Boost must now specify "python27" or "python36" as a
 component to look for, not just "python" or "python2" or "python3";
 guessing other build systems will be impacted as well.

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Ticket URL: <https://trac.macports.org/ticket/56294#comment:10>
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