Old syntax suggestion

[torhovland]

Fixes #13868.

The build error in the issue will now include a suggestion:

   Compiling zerocopy v0.8.0-alpha.9
error: the `cargo::` syntax for build script output instructions was added in Rust 1.77.0, but the minimum supported Rust version of `zerocopy v0.8.0-alpha.9` is 1.56.0.
Consider using the old `cargo:` syntax in front of `rustc-check-cfg=`.
See https://doc.rust-lang.org/cargo/reference/build-scripts.html#outputs-of-the-build-script for more information about build script outputs.

The suggestion is only included for reserved prefixes.

A test has been added.

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @weihanglo (or someone else) some time within the next two weeks.

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[torhovland]

@rustbot review

[torhovland]

OK, I squashed a couple commits. Hope that's better.

[weihanglo]

@bors r+

Thanks!

(To me I might also make git commit messages clearer. You can see how people did it in this repo. Let's don't bother on this at this moment and move on 👍🏾)

[bors]

:pushpin: Commit 3ea3638c1b2718d01dab8fd6d115b58d40134032 has been approved by weihanglo

It is now in the queue for this repository.

[bors]

:hourglass: Testing commit 3ea3638c1b2718d01dab8fd6d115b58d40134032 with merge 1fec08999177fb768e468653dd05ff8bf8e250b9...

[bors]

:sunny: Test successful - checks-actions Approved by: weihanglo Pushing 1fec08999177fb768e468653dd05ff8bf8e250b9 to master...