fix(build): make portable checks target-aware

Select a portable architecture from the compiler target instead of forcing x86-64-v3 on every platform. On macOS, only enable Homebrew OpenMP when its header and library actually exist, preserving the dependency-free fallback.

c/tests/test_makefile_platform.py

@@ -11,6 +11,24 @@ MAKE = shutil.which("make")
 
 @unittest.skipUnless(MAKE, "make is required")
 class MakefilePlatformTests(unittest.TestCase):
+    def _dry_run(self, target, triplet, **variables):
+        args = [
+            MAKE,
+            "--no-print-directory",
+            "-B",
+            "-n",
+            target,
+            f"TRIPLET={triplet}",
+        ]
+        args.extend(f"{name}={value}" for name, value in variables.items())
+        return subprocess.run(
+            args,
+            cwd=C_DIR,
+            text=True,
+            capture_output=True,
+            check=True,
+        )
+
     def test_windows_nt_without_uname_selects_mingw_build(self):
         env = os.environ.copy()
         env["OS"] = "Windows_NT"
@@ -29,6 +47,29 @@ class MakefilePlatformTests(unittest.TestCase):
         self.assertIn("-fopenmp", result.stdout)
         self.assertIn("-static", result.stdout)
 
+    def test_portable_build_uses_target_architecture(self):
+        cases = (
+            ("x86_64-unknown-linux-gnu", "-march=x86-64-v3"),
+            ("aarch64-unknown-linux-gnu", "-march=armv8-a"),
+            ("powerpc64le-unknown-linux-gnu", "-mcpu=power8"),
+        )
+
+        for triplet, expected_flag in cases:
+            with self.subTest(triplet=triplet):
+                result = self._dry_run("portable", triplet)
+                self.assertIn(expected_flag, result.stdout)
+
+    def test_darwin_portable_build_does_not_force_x86_architecture(self):
+        missing_libomp = "/colibri-test/missing-libomp"
+        result = self._dry_run(
+            "portable", "arm64-apple-darwin", OMPDIR=missing_libomp
+        )
+
+        self.assertIn("clang -O3", result.stdout)
+        self.assertNotIn("-mcpu=x86-64-v3", result.stdout)
+        self.assertNotIn(missing_libomp, result.stdout)
+        self.assertNotIn("-fopenmp", result.stdout)
+
 
 if __name__ == "__main__":
     unittest.main()