GH-49841: [C++] Fix macOS 12.3 SDK build by using __cpp_lib_int_pow2

[1fanwang]

Rationale for this change

Building Arrow C++ against the macOS 12.3 SDK fails:

error: no member named 'log2p1' in namespace 'std'

Apple clang shipped std::log2p1, the pre-standard name for std::bit_width, in
older SDKs. Arrow keeps a workaround that calls it when __cpp_lib_bitops is
undefined, but that is the wrong feature-test macro. It covers popcount, rotl and
friends, while bit_width and has_single_bit are covered by __cpp_lib_int_pow2.
Probing both SDKs shows why that matters:

SDK	bitops	int_pow2	bit_width	log2p1
11.3	undefined	undefined	absent	present
12.3	undefined	202002	present	absent

__cpp_lib_bitops is undefined on both, so on 12.3 the guard selects the one name
that is missing. Before this change Arrow cannot be built on that SDK; after it, it
builds.

Closes #49841.

What changes are included in this PR?

Switch the discriminator to __cpp_lib_int_pow2 at the five sites using this
workaround, keeping the __apple_build_version__ condition and every fallback
branch unchanged. One of the five guards has_single_bit rather than bit_width. It
still compiled on 12.3 because its fallback is a manual power-of-two check, but the
macro is wrong there for the same reason, so it is corrected too.

Are these changes tested?

Each affected translation unit was compiled against both SDKs, before and after.

$ curl -fsSL -o s.tar.xz https://github.com/alexey-lysiuk/macos-sdk/releases/download/12.3/MacOSX12.3.tar.xz
$ mkdir /tmp/s && tar -xJf s.tar.xz -C /tmp/s && cd cpp/src
$ CC="clang++ -std=gnu++20 -isysroot /tmp/s/MacOSX12.3.sdk -I . -fsyntax-only"
$ F=parquet/encoder.cc

$ git switch main && $CC $F
./arrow/util/bit_util.h:149:15: error: no member named 'log2p1' in namespace 'std'
2 errors generated.

$ git switch - && $CC $F && echo compiled successfully
compiled successfully

All five compile on both SDKs after the change. Compiling on 11.3 is itself proof
the log2p1 branch is still selected there, since bit_width does not exist in that
SDK.

The covering test target builds and passes on this machine's current SDK:

[  PASSED  ] 204 tests.

Are there any user-facing changes?

No API change. Arrow C++ now builds against macOS SDK 12.3 and later.

• GitHub Issue: [C++] error: no member named 'log2p1' in namespace 'std' #49841

[github-actions]

⚠️ GitHub issue #49841 has been automatically assigned in GitHub to PR creator.

[Copilot]

Pull request overview

This PR fixes Arrow C++ builds against the macOS 12.3 SDK by switching the Apple-clang-specific workaround selector from __cpp_lib_bitops to the correct feature-test macro __cpp_lib_int_pow2, which matches the availability of std::bit_width / std::has_single_bit (and avoids incorrectly selecting the std::log2p1 fallback where it no longer exists).

Changes:

• Update preprocessor guards at five call sites to use __cpp_lib_int_pow2 instead of __cpp_lib_bitops.
• Add brief explanatory comments indicating the correct feature-test macro for std::bit_width / std::has_single_bit.
• Keep all fallback branches and Apple-clang gating (__apple_build_version__) unchanged.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
cpp/src/parquet/encoder.cc	Uses __cpp_lib_int_pow2 to correctly choose between std::log2p1 and std::bit_width on Apple clang + varying SDKs.
cpp/src/parquet/chunker_internal.cc	Fixes the SDK-dependent selection for computing target_bits to avoid selecting missing std::log2p1.
cpp/src/arrow/util/rle_encoding_test.cc	Aligns the test’s feature selection with the correct macro so it matches the production behavior across SDKs.
cpp/src/arrow/util/bit_util.h	Fixes the Log2 helper’s Apple-clang workaround guard to correctly detect std::bit_width availability.
cpp/src/arrow/util/align_util.h	Corrects the guard for std::has_single_bit selection to use __cpp_lib_int_pow2.

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[Copilot]

🟢 Approval recommended

The changes are narrowly scoped to correcting the feature-test macro used for <bit> APIs on Apple SDKs and match the stated build failure and resolution without introducing behavioral changes beyond the intended conditional selection.

Review details

• Files reviewed: 5/5 changed files
• Comments generated: 0 new
• Review effort level: Lite