feat(build): improve dependency error reporting in autogen.sh #30646

Improve error reporting in autogen.sh by redirecting the autoreconf dependency check's error message to stderr.

Impact

This PR modifies autogen.sh in the root directory.

• No change to the functional behavior of these files.
• Enhances error reporting by directing error messages to stderr, aligning with best practices.

Background

1. 9 years ago: Added the check for the autoreconf dependency to the autogen.sh build script. This check avoids a not found error because the next line runs autoreconf.

   • Original error message: ./autogen.sh: 9: ./autogen.sh: autoreconf: not found
   • Updated error message: configuration failed, please install autoconf first
   • Link to commit to the commit.

2. 5 years ago: Updated the check from which to command -v to improve portability and reliability as part of a large lint refactor.

   • Link to commit to that commit.

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Please let me know if it's preferred to squash the three commits.

The changes to secp256k1 should be done in https://github.com/bitcoin-core/secp256k1, and the changes to minisketch should be done in https://github.com/sipa/minisketch.

I assume the autogen.sh will go away with the imminent cmake migration?

@sipa understood about secp256k1 and minisketch.

Setting as draft to remove those changes from this PR.

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🚧 At least one of the CI tasks failed. _{Debug: https://github.com/bitcoin/bitcoin/runs/28715049878}

<details><summary>Hints</summary>

Make sure to run all tests locally, according to the documentation.

The failure may happen due to a number of reasons, for example:

• Possibly due to a silent merge conflict (the changes in this pull request being incompatible with the current code in the target branch). If so, make sure to rebase on the latest commit of the target branch.

• A sanitizer issue, which can only be found by compiling with the sanitizer and running the affected test.

• An intermittent issue.

Leave a comment here, if you need help tracking down a confusing failure.

</details>

Taking back out of draft. PR is now the single change to the main autogen.sh.

Tend toward NACK. No need to apply style fixes to a file that will be deleted in 10 days (or whenever the branch-off happens and the cmake pull is reviewed sufficiently)

Thanks for reviewing. I'll close the PR as won't do.