[MacPorts] #59834: Boost: update to 1.76.0

[MacPorts]

#59834: Boost: update to 1.76.0
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  Reporter:  michaelld    |      Owner:  mascguy
      Type:  enhancement  |     Status:  assigned
  Priority:  Normal       |  Milestone:
 Component:  ports        |    Version:
Resolution:               |   Keywords:
      Port:  boost        |
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Changes (by mascguy):

 * type:  update => enhancement


Old description:

> Boost 1.76.0 was released on 20210416:
> https://www.boost.org/users/history/version_1_76_0.html

New description:

 Tracking ticket to cover various improvements/refactoring for our Boost
 ports, once the following PR has been merged:

 [https://github.com/macports/macports-ports/pull/11185 PR 11185 - Add
 boost171 and boost176 ports, with isolated builds, and a new boost PG to
 handle configuration]

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Ticket URL: <https://trac.macports.org/ticket/59834#comment:31>
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