Add support for detecting free-threaded Python on Windows

This does a couple of things:
1. Scrape the `Py_GIL_DISABLED` sysconfig var, which is the best way as of today
   to determine whether the target interpreter was built with `--disable-gil`
2. link against the correct libpython
3. On Windows, work around a known issue in the python.org installer with a
   missing define in `pyconfig.h`, which the CPython devs have said is unlikely
   to be fixed since headers are shared between the regular and free-threaded
   builds in a single NSIS installer.

mesonbuild/dependencies/python.py

@@ -29,6 +29,7 @@ class PythonIntrospectionDict(TypedDict):
         install_paths: T.Dict[str, str]
         is_pypy: bool
         is_venv: bool
+        is_freethreaded: bool
         link_libpython: bool
         sysconfig_paths: T.Dict[str, str]
         paths: T.Dict[str, str]
@@ -91,6 +92,7 @@ def __init__(self, name: str, command: T.Optional[T.List[str]] = None,
             'install_paths': {},
             'is_pypy': False,
             'is_venv': False,
+            'is_freethreaded': False,
             'link_libpython': False,
             'sysconfig_paths': {},
             'paths': {},
@@ -146,6 +148,7 @@ def __init__(self, python_holder: 'BasicPythonExternalProgram', embed: bool):
         self.variables = python_holder.info['variables']
         self.paths = python_holder.info['paths']
         self.is_pypy = python_holder.info['is_pypy']
+        self.is_freethreaded = python_holder.info['is_freethreaded']
         # The "-embed" version of python.pc / python-config was introduced in 3.8,
         # and distutils extension linking was changed to be considered a non embed
         # usage. Before then, this dependency always uses the embed=True handling
@@ -161,6 +164,12 @@ def __init__(self, python_holder: 'BasicPythonExternalProgram', embed: bool):
         else:
             self.major_version = 2
 
+        # pyconfig.h is shared between regular and free-threaded builds in the
+        # Windows installer from python.org, and hence does not define
+        # Py_GIL_DISABLED correctly. So do it here:
+        if mesonlib.is_windows() and self.is_freethreaded:
+            self.compile_args += ['-DPy_GIL_DISABLED']
+
     def find_libpy(self, environment: 'Environment') -> None:
         if self.is_pypy:
             if self.major_version == 3:
@@ -220,7 +229,10 @@ def get_windows_link_args(self, limited_api: bool) -> T.Optional[T.List[str]]:
                 else:
                     if limited_api:
                         vernum = vernum[0]
-                    libpath = Path('libs') / f'python{vernum}.lib'
+                    if self.is_freethreaded:
+                        libpath = Path('libs') / f'python{vernum}t.lib'
+                    else:
+                        libpath = Path('libs') / f'python{vernum}.lib'
                     # For a debug build, pyconfig.h may force linking with
                     # pythonX_d.lib (see meson#10776). This cannot be avoided
                     # and won't work unless we also have a debug build of

mesonbuild/scripts/python_info.py

@@ -106,6 +106,9 @@ def links_against_libpython():
 if is_pypy:
     limited_api_suffix = suffix
 
+# Whether we're targeting a free-threaded CPython interpreter
+is_freethreaded = bool(variables.get('Py_GIL_DISABLED', False))
+
 print(json.dumps({
   'variables': variables,
   'paths': paths,
@@ -118,4 +121,5 @@ def links_against_libpython():
   'link_libpython': links_against_libpython(),
   'suffix': suffix,
   'limited_api_suffix': limited_api_suffix,
+  'is_freethreaded': is_freethreaded,
 }))