Take the C extension's include directories from sysconfig

`developer.c-extension` split `python-config --includes` into arguments,
which can't work when the installation path contains spaces, as the
relocated tree's does. That made the check fail on Linux, and on macOS
it repeated `developer.python-config`'s stale shebang failure without
compiling anything. Use the moved interpreter's `sysconfig.get_paths()`
instead, as build tools do; `developer.python-config` still checks
python-config itself.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: itamaro

relocatability.py

@@ -769,9 +769,14 @@ def _check_venv(self) -> str:
     def _check_c_extension(self) -> str:
         if self.snapshot is None:
             raise CheckSkipped("the startup probe did not complete")
-        config = self._python_config()
-        includes = self._run([config, "--includes"])
-        self._require_success(includes, "python-config --includes failed")
+        # Ask sysconfig, as build tools do. python-config's output can't
+        # be split into arguments when the path contains spaces.
+        paths = self.snapshot.get("config_paths") or {}
+        includes = [
+            f"-I{paths[name]}" for name in ("include", "platinclude") if paths.get(name)
+        ]
+        if not includes:
+            raise CheckFailed("sysconfig did not provide the include directories")
         ldshared = self.snapshot.get("ldshared")
         ext_suffix = self.snapshot.get("ext_suffix")
         if not isinstance(ldshared, str) or not ldshared:
@@ -824,7 +829,7 @@ def _check_c_extension(self) -> str:
             *shlex.split(ldshared),
             # e.g. -fPIC on Linux; LDSHARED doesn't include it.
             *shlex.split(self.snapshot.get("ccshared") or ""),
-            *shlex.split(includes.stdout),
+            *includes,
             os.fspath(source),
             "-o",
             os.fspath(extension),