use CFLAGS envvar instead of extra_compile_args

setup.py

@@ -51,12 +51,17 @@ def __init__(self, *args, **kwargs):
 
 libraries = []
 macros = []
-extra_compile_args = []
 
 if sys.platform == "win32":
     libraries.append("ws2_32")
     macros = [("__LITTLE_ENDIAN__", "1")]
-    extra_compile_args = ["/std:c++20"]
+    cflags = os.environ.get("CFLAGS")
+    cxx20flag = "/std:c++20"
+    if cflags is None:
+        cflags = cxx20flag
+    elif cxx20flag not in cflags:
+        cflags += " " + cxx20flag
+    os.environ["CFLAGS"] = cflags
 
 ext_modules = []
 if not PYPY and not os.environ.get("MSGPACK_PUREPYTHON"):
@@ -67,7 +72,6 @@ def __init__(self, *args, **kwargs):
             libraries=libraries,
             include_dirs=["."],
             define_macros=macros,
-            extra_compile_args=extra_compile_args,
         )
     )
 del libraries, macros