Add in-tree build backend.

MANIFEST.in

@@ -22,3 +22,4 @@ include *.py
 include *.release
 include *.sh
 include LICENSE
+include _build/*.py

_build/backend.py

@@ -0,0 +1,25 @@
+# -*- coding: utf-8 -*-
+import os
+from setuptools.build_meta import *
+import subprocess
+
+
+def netcdf_has_parallel_support():
+    netcdf4_dir = os.environ.get("NETCDF4_DIR")
+    ncconfig = os.path.join(netcdf4_dir, "bin", "nc-config") if netcdf4_dir else "nc-config"
+    process = subprocess.run([ncconfig, "--has-parallel4"], capture_output=True)
+    out = process.stdout.decode("utf-8").rstrip()
+    return out == "yes"
+
+
+def get_requires_for_build_editable(config_settings=None):
+    return ["mpi4py>=3.1"] if netcdf_has_parallel_support() else []
+
+
+def get_requires_for_build_wheel(config_settings=None):
+    return ["mpi4py>=3.1"] if netcdf_has_parallel_support() else []
+
+
+def get_requires_for_build_sdist(config_settings=None):
+    return ["mpi4py>=3.1"] if netcdf_has_parallel_support() else []
+

pyproject.toml

@@ -1,11 +1,12 @@
 [build-system]
 requires = [
     "Cython>=0.29",
-    "mpi4py>=3.1",
     "oldest-supported-numpy",
     "setuptools>=61",
+    "wheel"
 ]
-build-backend = "setuptools.build_meta"
+build-backend = "backend"
+backend-path = ["_build"]
 
 [project]
 name = "netCDF4"