[MacPorts] #53139: gcc7 update

[MacPorts]

#53139: gcc7 update
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  Reporter:  rouson  |      Owner:  ryandesign
      Type:  update  |     Status:  accepted
  Priority:  Normal  |  Milestone:
 Component:  ports   |    Version:
Resolution:          |   Keywords:
      Port:  gcc7    |
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Comment (by ryandesign):

 We need more than just a simple update here. The library version of one of
 the gcc libraries changed in the last version (or I noticed it in the last
 version), and as a result, we need to add a libgcc6 subport to the gcc6
 port to provide the previous library version and make gcc6 and all
 previous versions use it.

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Ticket URL: <https://trac.macports.org/ticket/53139#comment:3>
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