[MacPorts] #44810: gsl : Add gcc49 variant

[MacPorts]

#44810: gsl : Add gcc49 variant
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  Reporter:  dixiao@…     |      Owner:  dtakahashi@…
      Type:  enhancement  |     Status:  new
  Priority:  Normal       |  Milestone:
 Component:  ports        |    Version:
Resolution:               |   Keywords:
      Port:  gsl          |
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Comment (by dtakahashi@…):

 Thank you for the suggestion! Do you know if there are some documents of
 the compilers portgroup (in order not to depend on internal symbols)?

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Ticket URL: <https://trac.macports.org/ticket/44810#comment:3>
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