[MacPorts] #44381: graphviz, graphviz-devel: Add port:flex build dependency

[MacPorts]

#44381: graphviz, graphviz-devel: Add port:flex build dependency
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  Reporter:  michaelld                |      Owner:  ryandesign
      Type:  defect                   |     Status:  new
  Priority:  Normal                   |  Milestone:
 Component:  ports                    |    Version:  2.3.1
Resolution:                           |   Keywords:
      Port:  graphviz graphviz-devel  |
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Comment (by mascguy):

 Since we're in the process of updating graphviz, the $100,000 question is:
 Should we add a build dep for flex? If not, this ticket can be closed as
 "won't fix."

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Ticket URL: <https://trac.macports.org/ticket/44381#comment:8>
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