[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 macsforever2000@…):

 It would be nice to add other gcc variants including gcc45 (the current
 default for Macports scientific ports), gcc46 and gcc48.

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Ticket URL: <https://trac.macports.org/ticket/37423#comment:1>
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