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

[MacPorts]

#62011: opencv/opencv4: eliminate portfile duplication, via subport
-----------------------------+----------------------
  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 subports.
>
> Also add the following variants, which I missed when bringing both up to
> parity:
> * opencv: `openmp`
> * opencv4: `openni`
>
> Other notable cleanup that will be tackled during the reconciliation:
> * Universal use of `github` portgroup. Currently used for opencv4, but
> not for opencv/opencv3.
> * Fully implement variant `openmp`, which is effectively a stub at this
> point. (While OpenMP is being enabled via configure flag,
> `compiler.openmp_version` isn't being set. With the result being that
> OpenMP isn't found, effectively making this variant a no-op at the
> moment.)
>
> There is additional cleanup as well, but that covers the most notable
> items.

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 subports.

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

 Other notable cleanup that will be tackled during the reconciliation:
 * Universal use of `github` portgroup. Currently used for opencv4, but not
 for opencv/opencv3.
 * Adopt `cmake` portgroup, to reduce number of CMake flags needed.
 * Fully implement variant `openmp`, which is effectively a stub at this
 point. (While OpenMP is being enabled via configure flag,
 `compiler.openmp_version` isn't being set. With the result being that
 OpenMP isn't found, effectively making this variant a no-op at the
 moment.)

 There is additional cleanup as well, but that covers the most notable
 items.

--

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Ticket URL: <https://trac.macports.org/ticket/62011#comment:11>
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