fix: 判据改用 MCPP_TARGET_REQUESTED —— MCPP_TARGET 从不为空

    ld64.lld: error: undefined symbol: wcslen / strtoul / __error
    (本包自己的 conformance,本机 macOS)

MCPP_TARGET 在没给 --target 时用宿主填上,所以「非空」恒真,于是这个 stub 上了一条
**本机**链接线,而那里 SDK 完整的那份本来就在,我们这份把它遮蔽了。

MCPP_TARGET_REQUESTED 是同一个值未被填充的形态:恰好在这是一次普通本机构建时为空。

⭐ 太旧而不设这个变量的 mcpp 会读到空,而那对它是正确答案:它没有「目标侧来自图」
这回事,系统永远在链接线上。

本机双向复验:交叉到 aarch64-macos 供给 stub 并产出;用已发布的 2026.8.19.1 走本机
路径构建本包,不供给,通过。

Author: speak-agent

build.mcpp

@@ -74,8 +74,22 @@ int main() {
     // the only thing that can name one. When it was not — a native build, which
     // is what this package's own conformance suite does — the SDK is on the
     // link and is the complete answer.
-    const char* target = std::getenv("MCPP_TARGET");
-    if (target && *target)
+    // ⚠️ `MCPP_TARGET_REQUESTED` AND NOT `MCPP_TARGET`. The second is filled in
+    // with the host when nobody named a target, so it is never empty and the
+    // test was always true — which put this stub on a NATIVE link, where the
+    // SDK's complete one was already there and ours shadowed it:
+    //
+    //     ld64.lld: error: undefined symbol: wcslen / strtoul / __error
+    //
+    // The first is the value unfilled: empty exactly when this is an ordinary
+    // native build. See mcpp's `build_program.cppm` for why the two are
+    // different questions even when the triples are equal.
+    //
+    // ⭐ An mcpp too old to set it leaves it empty, and that is the right answer
+    // for such a build tool: it has no graph-supplied target side, so the
+    // system is always on the link.
+    const char* requested = std::getenv("MCPP_TARGET_REQUESTED");
+    if (requested && *requested)
         mcpp::link_search("port");
     mcpp::rerun_if_changed("port/libSystem.tbd");
     return 0;