abseil: Use the minimum C++ standard supported

[adamnovak]

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This changes the Abseil build to build for the minimum C++ standard that Abseil supports. Abseil is now forward but not backward-compatible, so this should be compatible with the most C++ standards for downstream applications.

[adamnovak]

It's not clear to me yet how to build this manually from a Github branch, instead of from edits made via brew tap and brew edit. I'll keep trying to see if I can get Brew to run my modified formula for me.

[carlocab]

I'm sympathetic with the desire to use an older C++ standard here; using C++17 makes things rather difficult for dependent software.

However, doing this will break dependents like or-tools. You can check this for yourself by building or-tools from source after making this change. But CC @Mizux in case there are workarounds for this.

If you need a version of Protobuf that doesn't require C++17, please use protobuf@21.

[carlocab]

I don't think there's a way to do this without breaking dependents such as or-tools that use/need C++17. Therefore, closing.

If there is a workaround for the above, please open a new pull request that implements it.

[adamnovak]

I don't think there's a way to do this without breaking dependents such as or-tools that use/need C++17. Therefore, closing.

I must be wrong about how Abseil's forward-compatibility is supposed to work, then. There was a commit over there that seemed to suggest you could (now) build C++17 code against an Abseil built on C++14 and have it work, if you let Abseil's CMake write its config header into the install. But maybe that's not actually true.

In that case, maybe the real answer is to expose somehow to user code somehow what C++ version Homebrew's Abseil is? Is there a way to ask that question about a package with brew?

[carlocab]

This is not currently supported, and I'm not sure doing so would be very useful since it would be Homebrew-specific.

If there were a standard way to do it used by other package managers then that would be a different story.

However, I think you can use CMake to inspect the properties of targets provided by Abseil to determine the C++ standard they were compiled with.