Use system CMake when available

Replace the unconditional build requirement of `cmake` with one that
is added only if system CMake is not available, in order to facilitate
building against system CMake when available.  This avoids unnecessary
dependency on some systems, and improves portability when the particular
target requires downstream patching of CMake.

While at it, also update the existing CMake check to use
`shutil.which()` which is more efficient than spawning a subprocess,
and update the error message to reference matching CMake version.

Author: mgorny

pyproject.toml

@@ -18,7 +18,6 @@
 [build-system]
 requires = ["wheel",
             "setuptools >= 30.3.0",
-            "cmake >= 3.16",
             "numpy",
             "nanobind >= 1.6"]
 build-backend = "setuptools.build_meta"

setup.py

@@ -21,6 +21,7 @@
 import os
 import sys
 import platform
+import shutil
 import subprocess
 import re
 from datetime import datetime, timezone
@@ -35,11 +36,9 @@ def __init__(self, name, sourcedir=''):
 
 class CMakeBuild(build_ext):
     def run(self):
-        try:
-            subprocess.check_output(['cmake', '--version'])
-        except OSError:
+        if shutil.which('cmake') is None:
             raise RuntimeError(
-                "CMake >= 3.12 must be installed to build the following extensions: " +
+                "CMake >= 3.16 must be installed to build the following extensions: " +
                 ", ".join(e.name for e in self.extensions))
 
         for ext in self.extensions:
@@ -89,6 +88,10 @@ def build_extension(self, ext):
 ds_version = re.sub('@DT@', dt.strftime('%Y%m%d'), ds_version)
 ds_version = re.sub('@HHMM@', 'dev' + dt.strftime('%H%M'), ds_version)
 
+setup_requires = []
+if shutil.which('cmake') is None:
+    setup_requires += ['cmake >= 3.16']
+
 setup(
     name='datasketches',
     version=ds_version,
@@ -104,6 +107,7 @@ def build_extension(self, ext):
     # may need to add all source paths for sdist packages w/o MANIFEST.in
     ext_modules=[CMakeExtension('datasketches','.')],
     cmdclass={'build_ext': CMakeBuild},
+    setup_requires=setup_requires,
     install_requires=['numpy'],
     zip_safe=False
 )