build: support config clang stdlib and fix default

[egolearner]

Greptile Summary

This PR introduces a user-configurable CLANG_USE_LIBCXX CMake option to control whether Clang uses libc++ or libstdc++, replacing the previous hard-coded -stdlib=libc++ flag. The default is ON on Apple platforms and OFF elsewhere, which aligns with platform conventions. The change also cleans up trailing whitespace in several function bodies.

Key changes:

• New CLANG_USE_LIBCXX option and derived CLANG_STDLIB_OPTION variable used across compile and link flags.
• Compile-flag generator expression and both strict/unstrict link-flag entries updated to use the variable.
• Whitespace-only cleanup throughout _target_link_libraries and helpers.

Issues found:

• When CLANG_STDLIB_OPTION is empty, the compile-flag generator expression gains a trailing semicolon (-fcolor-diagnostics;), which causes CMake to emit an empty string as a separate compiler argument — a potential build error on some Clang versions.
• CMAKE_CXX_COMPILER_ID MATCHES "Clang" uses regex and inadvertently matches "AppleClang". Because the downstream generator expressions use exact CXX_COMPILER_ID:Clang matching, CLANG_STDLIB_OPTION is set for Apple Clang users but silently never applied. Using STREQUAL "Clang" would make the intent explicit.
• The -stdlib flag is forwarded to C compilations (via C_COMPILER_ID:Clang), which is unnecessary since -stdlib is a C++ concept.

Confidence Score: 3/5

• The PR is mostly safe to merge but contains a generator-expression bug that can inject a blank compiler argument on Linux with LLVM Clang when libc++ is not selected.
• The core intent is sound and defaults are platform-appropriate. However, the empty-CLANG_STDLIB_OPTION trailing-semicolon issue can cause silent or noisy build failures for Linux/LLVM Clang users with the default OFF setting, and the MATCHES vs STREQUAL inconsistency makes the logic harder to maintain. These issues reduce confidence below a clean pass.
• cmake/bazel.cmake — generator expression at line 389 and the MATCHES condition at line 375 both need attention.

Important Files Changed

Filename

Overview

cmake/bazel.cmake

Adds a configurable CLANG_USE_LIBCXX option (default ON on Apple, OFF elsewhere) and a CLANG_STDLIB_OPTION variable used in compile/link flags. Key issues: empty CLANG_STDLIB_OPTION creates a trailing semicolon in the generator expression that can emit a blank compiler argument; MATCHES "Clang" unintentionally catches AppleClang while downstream generator expressions do not; -stdlib flag is unnecessarily applied to C compilations.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[CMake Configure] --> B{APPLE platform?}
    B -- Yes --> C[CLANG_USE_LIBCXX = ON default]
    B -- No --> D[CLANG_USE_LIBCXX = OFF default]
    C --> E{CMAKE_CXX_COMPILER_ID\nMATCHES Clang?\nnote: also matches AppleClang}
    D --> E
    E -- No --> F[CLANG_STDLIB_OPTION = empty string]
    E -- Yes --> G{CLANG_USE_LIBCXX?}
    G -- ON --> H[CLANG_STDLIB_OPTION = -stdlib=libc++]
    G -- OFF --> I{APPLE?}
    I -- Yes --> F
    I -- No --> J[CLANG_STDLIB_OPTION = -stdlib=libstdc++]
    H --> K[Applied via generator expressions\nC_COMPILER_ID:Clang compile flags\nCXX_COMPILER_ID:Clang link flags]
    J --> K
    F --> L[⚠️ Empty value → trailing semicolon\nin generator expression → blank arg]
    K --> M[Build targets use CLANG_STDLIB_OPTION]

Loading

_{Last reviewed commit: a217750}

Greptile also left 3 inline comments on this PR.

[egolearner]

#214 add clang CI and will track ut problem there.