fix: avoid dyld env injection on macos

Author: Sunrisepeak

.agents/docs/2026-05-26-build-error-output-optimization-plan.md

@@ -188,7 +188,7 @@ std::string prepend_path_list(std::string_view key,
 | 平台 | 变量 | 说明 |
 |------|------|------|
 | Linux | `LD_LIBRARY_PATH` | 当前实际需要，替代 `toolenv` |
-| macOS | `DYLD_LIBRARY_PATH` | 仅给 mcpp 启动的 ninja/clang 子进程使用；如后续不需要可返回空 |
+| macOS | 空 | 不设置 `DYLD_LIBRARY_PATH`；它会影响 ninja 自身和系统 framework 的 dyld 解析，依赖 toolchain/rpath |
 | Windows | `PATH` | 如果私有工具链 DLL 需要搜索路径，按 `;` prepend；当前可先空实现 |
 
 `NinjaBackend::build()` 中：

src/platform/env.cppm

@@ -112,7 +112,10 @@ std::string runtime_library_path_key() {
 #if defined(_WIN32)
     return "PATH";
 #elif defined(__APPLE__)
-    return "DYLD_LIBRARY_PATH";
+    // DYLD_LIBRARY_PATH affects every executable launched by ninja, including
+    // ninja itself, and can make macOS system frameworks load an incompatible
+    // private libc++/libc++abi. Keep macOS on toolchain-provided rpaths.
+    return "";
 #elif defined(__linux__)
     return "LD_LIBRARY_PATH";
 #else