Use "parallel" config setting and 4 as defaults

Author: radarhere

setup.py

@@ -21,7 +21,15 @@
 from setuptools import Extension, setup
 from setuptools.command.build_ext import build_ext
 
-ParallelCompile("MAX_CONCURRENCY").install()
+configuration: dict[str, list[str]] = {}
+
+# parse configuration from _custom_build/backend.py
+while sys.argv[-1].startswith("--pillow-configuration="):
+    _, key, value = sys.argv.pop().split("=", 2)
+    configuration.setdefault(key, []).append(value)
+
+default = int(configuration.get("parallel", ["4"])[-1])
+ParallelCompile("MAX_CONCURRENCY", default).install()
 
 
 def get_version() -> str:
@@ -30,9 +38,6 @@ def get_version() -> str:
         return f.read().split('"')[1]
 
 
-configuration: dict[str, list[str]] = {}
-
-
 PILLOW_VERSION = get_version()
 AVIF_ROOT = None
 FREETYPE_ROOT = None
@@ -1047,11 +1052,6 @@ def debug_build() -> bool:
 ]
 
 
-# parse configuration from _custom_build/backend.py
-while sys.argv[-1].startswith("--pillow-configuration="):
-    _, key, value = sys.argv.pop().split("=", 2)
-    configuration.setdefault(key, []).append(value)
-
 try:
     setup(
         cmdclass={"build_ext": pil_build_ext},