Python port group overwrites CMAKE_MAKE_PROGRAM, breaks configure in opencv4

[Steven Smith]

Bothe the cmake and python port groups are necessary to build opencv4 and its python wrapper.

See https://github.com/macports/macports-ports/pull/6261#discussion_r382050271

What Portfile code will work to use the original cmake flags if port group python is used?

> On Jul 13, 2020, at 19:04, Joshua Root <jmr at macports.org> wrote:
> 
> The python portgroup is designed to aid installing software with
> setup.py. If your software is not installed that way, don't use the
> python portgroup.
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