fix CI? (mac and win pass; linux fails)

Author: t-kalinowski

build.rs

@@ -2,6 +2,13 @@ fn main() {
     // Keep extension-module link flags for the cdylib artifact.
     pyo3_build_config::add_extension_module_link_args();
 
+    // Skip explicit libpython linking when building the extension module itself.
+    // Manylinux wheels do not ship a linkable libpython, so the extra link flags
+    // cause the build to fail; tests/binaries still link for host runs.
+    if std::env::var_os("PYO3_BUILD_EXTENSION_MODULE").is_some() {
+        return;
+    }
+
     // Link tests/binaries against the discovered Python shared library so
     // `cargo test` works without extra environment flags.
     let cfg = pyo3_build_config::get();