[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@…):

 When built with the default hdf5 variant, it depends on hdf5-18 which has
 gcc4x variants. So I assume that silo would need to be built with the same
 gcc too. So it should match the hdf5-18 port which has variants on gcc44,
 gcc45, gcc46, and gcc47. I think gcc44 can be skipped though.

 Yes, having a gcc portgroup would be nice. But I'm unsure how hard that
 would be to make for all cases.

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Ticket URL: <https://trac.macports.org/ticket/37423#comment:4>
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