[MacPorts] #47578: mkvtoolnix: upgrade to 7.8.0

[MacPorts]

#47578: mkvtoolnix: upgrade to 7.8.0
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  Reporter:  mojca@…     |      Owner:  macports-tickets@…
      Type:  update      |     Status:  new
  Priority:  Normal      |  Milestone:
 Component:  ports       |    Version:
Resolution:              |   Keywords:
      Port:  mkvtoolnix  |
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Comment (by ryandesign@…):

 `BOOST_CODECVT_DO_LENGTH_CONST` was defined in utf8_codecvt_facet.hpp in
 boost 1.57.0 but this macro was
 [https://github.com/boostorg/detail/commit/1fef8494fec86d23e557007343036c3d8a5b0ace
 eliminated] for boost 1.58.0.

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Ticket URL: <https://trac.macports.org/ticket/47578#comment:2>
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