[Build] Skip spinloop extension on Python < 3.11 (#44783)

Signed-off-by: Jasen2201 <yajizhan@amd.com>

CMakeLists.txt

@@ -114,20 +114,23 @@ endif()
 # CPU builds define the target before the early return)
 # This extension requires SABI 3.11 since it relies on Py_buffer support. Loading
 # failure is handled gracefully on vLLM side for lower Python versions.
+# Skip the target entirely on Python < 3.11 so the build doesn't break.
 #
-set(VLLM_SPINLOOP_EXT_SRC "csrc/spinloop.cpp")
-set(SPINLOOP_COMPILE_FLAGS "")
-if(CMAKE_SYSTEM_PROCESSOR MATCHES "x86_64|amd64")
-  list(APPEND SPINLOOP_COMPILE_FLAGS "-mmwaitx")
+if(Python_VERSION VERSION_GREATER_EQUAL "3.11")
+  set(VLLM_SPINLOOP_EXT_SRC "csrc/spinloop.cpp")
+  set(SPINLOOP_COMPILE_FLAGS "")
+  if(CMAKE_SYSTEM_PROCESSOR MATCHES "x86_64|amd64")
+    list(APPEND SPINLOOP_COMPILE_FLAGS "-mmwaitx")
+  endif()
+  define_extension_target(
+    spinloop
+    DESTINATION vllm
+    LANGUAGE CXX
+    SOURCES ${VLLM_SPINLOOP_EXT_SRC}
+    COMPILE_FLAGS ${SPINLOOP_COMPILE_FLAGS}
+    USE_SABI 3.11
+    WITH_SOABI)
 endif()
-define_extension_target(
-  spinloop
-  DESTINATION vllm
-  LANGUAGE CXX
-  SOURCES ${VLLM_SPINLOOP_EXT_SRC}
-  COMPILE_FLAGS ${SPINLOOP_COMPILE_FLAGS}
-  USE_SABI 3.11
-  WITH_SOABI)
 
 #
 # Forward the non-CUDA device extensions to external CMake scripts.

setup.py

@@ -1086,7 +1086,8 @@ if _is_cuda() or _is_hip():
     # copying the relevant .py files from the source repository.
     ext_modules.append(CMakeExtension(name="vllm.triton_kernels", optional=True))
 
-ext_modules.append(CMakeExtension(name="vllm.spinloop"))
+if sys.version_info >= (3, 11):
+    ext_modules.append(CMakeExtension(name="vllm.spinloop"))
 
 if _is_hip():
     ext_modules.append(CMakeExtension(name="vllm._rocm_C"))