add mac flags for making extension

Author: rickeylev

tests/venv_site_packages_libs/ext_with_libs/BUILD.bazel

@@ -36,14 +36,24 @@ cc_library(
 # todo: copy more from py_extension in local_toolchains
 cc_shared_library(
     name = "adder",
+    # Necessary for several reasons:
+    # 1. Ensures the output doesn't include increment itself (avoids ODRs)
+    # 2. Adds -lincrement (DT_NEEDED for libincrement.so)
+    # 3. Ensures libincrement.so is available at link time to satisfy (2)
     dynamic_deps = [":increment"],
-    shared_lib_name = select({
-        "//conditions:default": "adder.so",
-    }),
+    shared_lib_name = "adder.so",
     user_link_flags = [
         # NOTE: cc_shared_library adds Bazelized rpath entries, too.
         "-Wl,-rpath,$ORIGIN/libs",
-    ],
+    ] + select({
+        "@platforms//os:osx": [
+            "-undefined",
+            "dynamic_lookup",
+            "-Wl,-exported_symbol",
+            "-Wl,_PyInit_adder",
+        ],
+        "//conditions:default": [],
+    }),
     deps = [":adder_impl"],
 )