opencv: use C++17

[ZhongRuoyu]

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C++17 is necessary for OpenCV to use Protobuf, due to: https://github.com/opencv/opencv/blob/4.9.0/cmake/OpenCVFindProtobuf.cmake#L81-L88

Fixes https://github.com/orgs/Homebrew/discussions/5138.

[chenrui333]

would test also need a change?

homebrew-core/Formula/o/opencv.rb

Line 173 in 907b5a2

system ENV.cxx, "-std=c++11", "test.cpp", "-I#{include}/opencv4", "-o", "test"

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