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

Lee Modules/posixmodule.c y el precedente de comprobación de disponibilidad en gh-97897; después, reproduce el escenario de configuración y compilación de Xcode 27/macOS 26 descrito en el informe. Se considera terminado cuando dup3() y pipe2() se gestionan de forma segura al compilar contra el SDK más reciente, pero ejecutarse en versiones anteriores de macOS, y las comprobaciones relevantes de compilación o de ejecución pasan correctamente.