gh-156764: Fix safety configure option defaults

[lpyu001]

Preserve the default enabled safety configuration while correctly handling enable, disable, and invalid values for both safety options.

• Issue: configure mishandles the default and --disable-* forms of the safety options #156764

[lpyu001]

Hi, could you take a look at this PR when you get a chance @corona10

[read-the-docs-community]

Documentation build overview

📚 cpython-previews | 🛠️ Build #35169172 | 📁 Comparing f40d7d4 against main (a6b4aa6)

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3 files changed

± using/configure.html
± whatsnew/3.14.html
± whatsnew/changelog.html

[lpyu001]

I've updated this PR. Both options now stay disabled by default, as decided in gh-124064.

• configure.ac / configure: use $enableval, so --disable-safety and --disable-slower-safety no longer enable the options. Invalid values are rejected.
• Doc/using/configure.rst and Doc/whatsnew/3.14.rst: document --enable-safety (off by default) instead of the old --disable-safety.

This affects 3.14 and 3.15 as well, so it should be backported.

[lpyu001]

cc @AA-Turner