[MacPorts] #54198: upgrade: port:gcc7

[MacPorts]

#54198: upgrade: port:gcc7
---------------------+------------------------
  Reporter:  RJVB    |      Owner:  ryandesign
      Type:  update  |     Status:  accepted
  Priority:  Normal  |  Milestone:
 Component:  ports   |    Version:
Resolution:          |   Keywords:  haspatch
      Port:  gcc7    |
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Comment (by RJVB):

 Why not update port:gcc7 and port:libgcc-devel to 7.1.0 (possibly
 introducing the libc++ variant as well) so people can start using the gcc7
 release version (and its libc++ support)?

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Ticket URL: <https://trac.macports.org/ticket/54198#comment:14>
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