[MacPorts] #62011: opencv/opencv4: eliminate portfile duplication, via subport

[MacPorts]

#62011: opencv/opencv4: eliminate portfile duplication, via subport
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  Reporter:  mascguy         |      Owner:  mascguy
      Type:  enhancement     |     Status:  assigned
  Priority:  Normal          |  Milestone:
 Component:  ports           |    Version:
Resolution:                  |   Keywords:
      Port:  opencv opencv4  |
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Description changed by mascguy:

Old description:

> Now that the two OpenCV-related ports have been brought up to parity,
> there are very few differences between them. Eliminate the portfile
> duplication, via a subport.

New description:

 Now that the two OpenCV-related ports have been brought up to parity,
 there are very few differences between them. Eliminate the portfile
 duplication, via a subport.

 If possible, also add the following variants, which I missed when bringing
 both up to parity:
 * opencv: `openmp`
 * opencv4: `openni`

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Ticket URL: <https://trac.macports.org/ticket/62011#comment:2>
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