[MacPorts] #56884: gerbil: use qt5 portgroup

[MacPorts]

#56884: gerbil: use qt5 portgroup
------------------------+------------------------
 Reporter:  ryandesign  |      Owner:  neverpanic
     Type:  defect      |     Status:  assigned
 Priority:  Normal      |  Milestone:
Component:  ports       |    Version:
 Keywords:              |       Port:  gerbil
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 gerbil should use the qt5 portgroup, rather than declaring a dependency
 explicitly on `port:qt5`. This will allow the correct older qt5X port to
 be used on older systems.

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Ticket URL: <https://trac.macports.org/ticket/56884>
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