[MacPorts] #47553: compilers portgroup: add option for gcc5

[MacPorts]

#47553: compilers portgroup: add option for gcc5
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 Reporter:  ryandesign@…  |      Owner:  sean@…
     Type:  enhancement   |     Status:  new
 Priority:  Normal        |  Milestone:
Component:  ports         |    Version:
 Keywords:                |       Port:  compilers
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 The compilers portgroup needs to add an option for gcc5, and in the future
 gcc6.

 This points out a defect in the portgroup, in which the gcc_versions
 variable assumes the first digit of the version number is "4", and
 similarly that the clang_versions variable assumes the first digit is "3".

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Ticket URL: <https://trac.macports.org/ticket/47553>
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