[MacPorts] #68849: root6 no longer has gsl

[MacPorts]

#68849: root6 no longer has gsl
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  Reporter:  patrick-macgregor  |      Owner:  cjones051073
      Type:  defect             |     Status:  assigned
  Priority:  Normal             |  Milestone:
 Component:  ports              |    Version:
Resolution:                     |   Keywords:
      Port:  root6              |
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Comment (by cjones051073):

 I remove the variant as upstream have removed the option to use a shared
 GSL installation

 ```
 CMake Error at cmake/modules/RootBuildOptions.cmake:413 (message):
   >>> Option 'gsl_shared' is no longer supported in ROOT 6.30.03.
 ```

 It is true though I also disabled the mathmore library at the same time.
 Let me take a look...

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Ticket URL: <https://trac.macports.org/ticket/68849#comment:4>
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