Allow restriction of number of threads when building pytket using cma…

…ke. (#1180)

build-without-conan.md

@@ -255,3 +255,7 @@ This needs the shared `tklog` and `tket` libraries to be installed.
 cd ${TKET_DIR}/pytket
 NO_CONAN=1 pip install -v -e .
 ```
+
+(You can use the environment variable `PYTKET_CMAKE_N_THREADS` to restrict the
+number of threads used in the above command; otherwise one thread per CPU is
+used.)

pytket/setup.py

@@ -65,7 +65,12 @@ def run(self):
             ["cmake", f"-DCMAKE_INSTALL_PREFIX={install_dir}", os.pardir], cwd=build_dir
         )
         subprocess.run(
-            ["cmake", "--build", os.curdir, f"-j{multiprocessing.cpu_count()}"],
+            [
+                "cmake",
+                "--build",
+                os.curdir,
+                f"-j{os.getenv('PYTKET_CMAKE_N_THREADS', multiprocessing.cpu_count())}",
+            ],
             cwd=build_dir,
         )
         subprocess.run(["cmake", "--install", os.curdir], cwd=build_dir)