[MacPorts] #58856: Boost: update to 1.71.0

[MacPorts]

#58856: Boost: update to 1.71.0
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 Reporter:  michaelld    |      Owner:  (none)
     Type:  enhancement  |     Status:  new
 Priority:  Normal       |  Milestone:
Component:  ports        |    Version:
 Keywords:               |       Port:  boost
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 Boost version 1.71.0 was release "August 19th, 2019 15:31 GMT". .. time to
 see about updating it within MP even though we just pushed the update to
 1.70.0 within the last week or so! I'll get the initial PR in place for
 the update, hopefully later this week, and verify that this update is
 "simple" / "clean" with the ports that I use that depend on it.

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Ticket URL: <https://trac.macports.org/ticket/58856>
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