[CI/Build]: make it possible to build with a free-threaded interpreter

For Python 3.13/3.14, free-threaded Python does not support using the
Limited C API and the Stable ABI.

Without this change, the build ends with:
```
  In file included from /path/to/vllm/csrc/core/registration.h:3,
                   from /path/to/vllm/csrc/cpu/torch_bindings.cpp:3:
  /..../include/python3.14t/Python.h:51:4: error: #error "The limited API is not currently supported in the free-threaded build"
```

Signed-off-by: Ralf Gommers <[email protected]>

Author: rgommers

cmake/utils.cmake

@@ -495,7 +495,13 @@ function (define_extension_target MOD_NAME)
     set(SOABI_KEYWORD "")
   endif()
 
-  if (ARG_USE_SABI)
+  run_python(IS_FREETHREADED_PYTHON
+    "import sysconfig; is_free = bool(sysconfig.get_config_var(\"Py_GIL_DISABLED\")); print(f'{1 if is_free else 0}')"
+    "Failed to determine whether interpreter is free-threaded")
+
+  # Free-threaded Python doesn't yet support the stable ABI (see PEP 803/809),
+  # so avoid using the stable ABI under free-threading only.
+  if (ARG_USE_SABI AND NOT IS_FREETHREADED_PYTHON)
     Python_add_library(${MOD_NAME} MODULE USE_SABI ${ARG_USE_SABI} ${SOABI_KEYWORD} "${ARG_SOURCES}")
   else()
     Python_add_library(${MOD_NAME} MODULE ${SOABI_KEYWORD} "${ARG_SOURCES}")

setup.py

@@ -10,6 +10,7 @@
 import shutil
 import subprocess
 import sys
+import sysconfig
 from pathlib import Path
 from shutil import which
 
@@ -74,9 +75,13 @@ def is_ninja_available() -> bool:
     return which("ninja") is not None
 
 
+def is_freethreaded():
+    return bool(sysconfig.get_config_var("Py_GIL_DISABLED"))
+
+
 class CMakeExtension(Extension):
     def __init__(self, name: str, cmake_lists_dir: str = ".", **kwa) -> None:
-        super().__init__(name, sources=[], py_limited_api=True, **kwa)
+        super().__init__(name, sources=[], py_limited_api=not is_freethreaded(), **kwa)
         self.cmake_lists_dir = os.path.abspath(cmake_lists_dir)