[OpenCV] Make the protobuf version a property

[ericriff]

I'm using a custom version of protobuf. I spent a bunch of time trying to debug a build error, turns out my protobuf versions were not in sync.
I can see this happening again when someone updates the version here. Current implementation is error prone.

By making the version a property we ensure that both the requirement and the build_requierement are in sync.

---

• I've read the guidelines for contributing.
• I've followed the PEP8 style guides for Python code in the recipes.
• I've used the latest Conan client version.
• I've tried at least one configuration locally with the
  conan-center hook activated.

[ghost]

I detected other pull requests that are modifying opencv/4.x recipe:

• opencv: add Intel IPP support #6843

This message is automatically generated by https://github.com/ericLemanissier/conan-center-conflicting-prs so don't hesitate to report issues/improvements there.

[uilianries]

Fair enough, as github diff hides untouched code, any dependencies bump could result in your case.

[ericriff]

It is hard to tell if this is making progress or if it is stuck.
CCI has been giving me a hard time haha

[prince-chrismc]

30hrs for OpenCV might be sensible... Maybe the team can look internal at Jenkins tomorrow? 🙏

[jgsogo]

Triggering that build again, sometimes it happens that the CI hangs 🤷

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About the PR, protobuf usually raise an error on runtime about mismatching versions to generate and use the generated protos, doesn't it? Nevermind, this will make the recipe more maintainable.

I would also add one extra check. Conan can override the protobuf version required here from downstream consumers, but the build-requires will always be the same. We can add something like the following to ensure that, at least when we are generating the packages, everything matches:

def build(self):
    if self.deps_cpp_info["protobuf"].version != self. _protobuf_version:
        raise ConanInvalidConfiguration("You are linking with protobuf/{} while your build require is protobuf/{}. Both need to be the same")

Typically this should go in validate(), but I think we cannot get the version there yet. Maybe we can improve the check, maybe patch versions are compatible.

[conan-center-bot]

All green in build 2 (460ce1c4e9264c5dcad60de71783cfae67f04598):

• opencv/4.1.2@:
  All packages built successfully! (All logs)

• opencv/4.5.0@:
  All packages built successfully! (All logs)

• opencv/4.5.1@:
  All packages built successfully! (All logs)

• opencv/4.5.2@:
  All packages built successfully! (All logs)

• opencv/4.5.3@:
  All packages built successfully! (All logs)