[Build] Do not add cmake/ninja dependencies when they are installed

pyproject.toml

@@ -1,8 +1,7 @@
 [build-system]
 # Should be mirrored in requirements/build.txt
+# Please do not add cmake or ninja here, they are handled in setup.py
 requires = [
-    "cmake>=3.26.1",
-    "ninja",
     "packaging>=24.2",
     "setuptools>=77.0.3,<80.0.0",
     "setuptools-scm>=8.0",

setup.py

@@ -70,6 +70,10 @@ def is_ninja_available() -> bool:
     return which("ninja") is not None
 
 
+def is_cmake_available() -> bool:
+    return which("cmake") is not None
+
+
 def is_url_available(url: str) -> bool:
     from urllib.request import urlopen
 
@@ -682,11 +686,18 @@ def _read_requirements(filename: str) -> list[str]:
         repackage_wheel if envs.VLLM_USE_PRECOMPILED else cmake_build_ext
     }
 
+setup_requires = []
+if not is_cmake_available():
+    setup_requires += ["cmake>=3.26.1"]
+if not is_ninja_available():
+    setup_requires += ["ninja"]
+
 setup(
     # static metadata should rather go in pyproject.toml
     version=get_vllm_version(),
     ext_modules=ext_modules,
     install_requires=get_requirements(),
+    setup_requires=setup_requires,
     extras_require={
         "bench": ["pandas", "datasets"],
         "tensorizer": ["tensorizer>=2.9.0"],