[MacPorts] #56433: update gmt4 to 4.5.17

[MacPorts]

#56433: update gmt4 to 4.5.17
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  Reporter:  fvaccari  |      Owner:  tenomoto
      Type:  update    |     Status:  assigned
  Priority:  Normal    |  Milestone:
 Component:  ports     |    Version:
Resolution:            |   Keywords:
      Port:  gmt4      |
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Comment (by fvaccari):

 Eh, [https://github.com/macports/macports-ports] is where I started
 from... I thought I did it, but apparently it's not the case. I had the
 best intentions to give back to MacPorts with this first (insignificant)
 contribution, but it looks like it's already beyond my capabilities. Will
 try to learn more, in due time.

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Ticket URL: <https://trac.macports.org/ticket/56433#comment:8>
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