iOS can't be built on macOS 26 / Xcode 26 build host

[freakboy3742]

Bug report

Bug description:

If you attempt to build the iOS distribution on macOS 26, building posixmodule.c on the ARM64 simulator fails with the following:

../Modules/posixmodule.c:11833:15: error: call to undeclared function 'dup3'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
 11833 |         res = dup3(fd, fd2, O_CLOEXEC);
       |               ^
../Modules/posixmodule.c:12701:11: error: call to undeclared function 'pipe2'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
 12701 |     res = pipe2(fds, O_CLOEXEC);
       |           ^
../Modules/posixmodule.c:12701:11: note: did you mean 'pipe'?
/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator26.4.sdk/usr/include/unistd.h:482:6: note: 'pipe' declared here
  482 | int      pipe(int [2]);
      |          ^
../Modules/posixmodule.c:12758:11: error: call to undeclared function 'pipe2'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
 12758 |     res = pipe2(fds, flags);
       |           ^

It appears that something has changed introducing linkable pipe2 and dup3 methods, but without a public header. This is enough to confuse configure into thinking the methods exist.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Other

Linked PRs

• gh-150443: Exclude explicit dup3 and pipe2 checks on iOS builds. #150444
• [3.15] gh-150443: Exclude explicit dup3 and pipe2 checks on iOS builds. (GH-150444) #150446
• [3.14] gh-150443: Exclude explicit dup3 and pipe2 checks on iOS builds. (GH-150444) #150447
• [3.13] gh-150443: Exclude explicit dup3 and pipe2 checks on iOS builds. (GH-150444) #150448