Bazel OpenCV snippets and useful patterns on Linux, Mac and Windows

For two years I have been using Bazel cmake() from rules_foreign_cc for hermetic OpenCV builds, but it turned out to be a disaster for a few reasons:

1. rules_foreign_cc has no concept of building from two related repositories, main OpenCV and contrib. There are some workarounds, but it is a waste of time.
2. Bazel with cmake() misteriously recompiles OpenCV for uknown reasons.
3. It takes 5 minutes or more on beefy Linux machines to build OpenCV.

It is much simpler to build whatever version is needed from scratch for different platforms. For Windows it requires config=windows and in .bazelproject add

build_flags:
  --config=windows

test_flags:
  --config=windows