macOS: Test availability for dup3() and pipe2() to prevent crashes when building against Xcode 27

Crash report

What happened?

This year's Apple OS releases (macOS/iOS/tvOS/watchOS/visionOS 27) add support for dup3() and pipe2(). When building for macOS using Xcode 27 beta but running on macOS 26, configure detects these functions, but Python crashes when it tries to use them. As a temporary fix, I'm passing ac_cv_func_dup3=no ac_cv_func_pipe2=no to configure. The real fix will be to use availability checks in Modules/posixmodule.c for dup3() and pipe2(), like gh-97897 does

Prototypes added to sys/unistd.h in the *27 SDK:

__API_AVAILABLE(macos(27.0), ios(27.0), tvos(27.0), watchos(27.0), visionos(27.0))
int     dup3(int, int, int);
__API_AVAILABLE(macos(27.0), ios(27.0), tvos(27.0), watchos(27.0), visionos(27.0))
int     pipe2(int [2], int);

@ned-deily @ronaldoussoren

CPython versions tested on:

3.13

Operating systems tested on:

macOS

Output from running 'python -VV' on the command line:

No response

Linked PRs

• gh-154718
• gh-155174
• gh-155202
• gh-155206
• gh-155211
• gh-155222

Leia Modules/posixmodule.c e o precedente de verificação de disponibilidade em gh-97897 e, em seguida, reproduza o cenário de configuração e build do Xcode 27/macOS 26 descrito no relatório. Considera-se concluído quando dup3() e pipe2() forem tratados com segurança ao compilar com o SDK mais recente, mas executar em versões mais antigas do macOS, e as verificações relevantes de build ou runtime forem aprovadas.