[MacPorts] #62011: opencv3/opencv4: eliminate portfile duplication via subports; simplify via portgroups; (was: opencv/opencv4: eliminate portfile duplication, via subports)

[MacPorts]

#62011: opencv3/opencv4: eliminate portfile duplication via subports; simplify via
portgroups;
------------------------------+----------------------
  Reporter:  mascguy          |      Owner:  mascguy
      Type:  enhancement      |     Status:  assigned
  Priority:  Normal           |  Milestone:
 Component:  ports            |    Version:
Resolution:                   |   Keywords:
      Port:  opencv3 opencv4  |
------------------------------+----------------------
Changes (by mascguy):

 * port:  opencv opencv4 => opencv3 opencv4


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.
> * 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.

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.

 Other notable cleanup that will be tackled during the reconciliation:
 * Universal use of `github` portgroup. Currently used for opencv4, but not
 for opencv3.
 * Adopt `cmake` portgroup, to reduce number of CMake flags needed.
 * If supported by OpenCV, simplify Python subports to generate Python
 bindings from built libs, rather than having to rebuild entire library.

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Ticket URL: <https://trac.macports.org/ticket/62011#comment:13>
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