Set Py_GIL_DISABLED=1 for free threading on Windows

When free threaded CPython is installed from the official Windows
installer it doesn't have the macro `Py_GIL_DISABLED` properly set
becuase its `pyconfig.h` file is shared across the co-installed default
build.

Define the macro when building free threaded Python extensions on
Windows so that each individual C API extension doesn't have to work
around this limitation.

See #4662

Author: colesbury

distutils/command/build_ext.py

@@ -23,7 +23,7 @@
 )
 from ..extension import Extension
 from ..sysconfig import customize_compiler, get_config_h_filename, get_python_version
-from ..util import get_platform, is_mingw
+from ..util import get_platform, is_mingw, is_freethreaded
 
 # An extension name is just a dot-separated list of Python NAMEs (ie.
 # the same as a fully-qualified module name).
@@ -333,6 +333,12 @@ def run(self):  # noqa: C901
         if os.name == 'nt' and self.plat_name != get_platform():
             self.compiler.initialize(self.plat_name)
 
+        # The official Windows free threaded Python installer doesn't set
+        # Py_GIL_DISABLED because its pyconfig.h is shared with the
+        # default build, so we need to define it here.
+        if os.name == 'nt' and is_freethreaded():
+            self.compiler.define_macro('Py_GIL_DISABLED', '1')
+
         # And make sure that any compile/link-related options (which might
         # come from the command-line or from the setup script) are set in
         # that CCompiler object -- that way, they automatically apply to

distutils/util.py

@@ -503,3 +503,7 @@ def is_mingw():
     get_platform() starts with 'mingw'.
     """
     return sys.platform == 'win32' and get_platform().startswith('mingw')
+
+def is_freethreaded():
+    """Return True if the Python interpreter is built with free threading support."""
+    return bool(sysconfig.get_config_var('Py_GIL_DISABLED'))