[MacPorts] #37423: silo: add +gcc47 variant

[MacPorts]

#37423: silo: add +gcc47 variant
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  Reporter:  johnsonsr@…  |      Owner:  macports-tickets@…
      Type:  enhancement  |     Status:  new
  Priority:  Normal       |  Milestone:
 Component:  ports        |    Version:  2.1.2
Resolution:               |   Keywords:  haspatch
      Port:  silo         |
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Comment (by johnsonsr@…):

 It would be nicer if MacPorts had the infrastructure to auto-generate the
 seven different GCC variants :) . If it's important, I can add it.

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Ticket URL: <https://trac.macports.org/ticket/37423#comment:2>
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