Support using system CMake and Ninja when available

Add the requirements for `cmake` and `ninja` PyPI packages only when
the respective system tools are not available, in order to permit
using the system tools instead.  This avoids unnecessary dependencies
on third-party executables, and permits users to benefit from greater
portability due to downstream patching of CMake.

pyproject.toml

@@ -9,8 +9,6 @@ requires = [
     "scikit-build>=0.13.1",
     "Cython>=0.24",
     "scipy>=0.16",
-    "cmake>=3.18",
-    "ninja"
 ]
 build-backend = "setuptools.build_meta"
 

setup.py

@@ -1,5 +1,6 @@
 import io
 import os.path
+import shutil
 
 from setuptools import find_packages
 from skbuild import setup
@@ -18,6 +19,13 @@ def exclude_non_implicit_cmake_files(cmake_manifest):
     return [f for f in cmake_manifest if "implicit" in f]
 
 
+setup_requires = []
+if shutil.which("cmake") is None:
+    setup_requires += ["cmake>=3.18"]
+if shutil.which("ninja") is None:
+    setup_requires += ["ninja"]
+
+
 setup(
     name="implicit",
     version="0.7.2",
@@ -44,6 +52,7 @@ def exclude_non_implicit_cmake_files(cmake_manifest):
         "Collaborative Filtering, Recommender Systems"
     ),
     packages=find_packages(),
+    setup_requires=setup_requires,
     install_requires=["numpy>=1.17.0", "scipy>=0.16", "tqdm>=4.27", "threadpoolctl"],
     cmake_process_manifest_hook=exclude_non_implicit_cmake_files,
 )