SCons: Fix MSVC D9025 warning overriding opt level

And cleanup disable_warnings too to avoid setting `/w` / `-w` multiple times.

(cherry picked from commit 9505f5fdd8ccc8954db874d0457d98c30bfed100)

methods.py

@@ -46,17 +46,12 @@ def disable_warnings(self):
     if self.msvc:
         # We have to remove existing warning level defines before appending /w,
         # otherwise we get: "warning D9025 : overriding '/W3' with '/w'"
-        warn_flags = ["/Wall", "/W4", "/W3", "/W2", "/W1", "/WX"]
-        self.Append(CCFLAGS=["/w"])
-        self.Append(CFLAGS=["/w"])
-        self.Append(CXXFLAGS=["/w"])
-        self["CCFLAGS"] = [x for x in self["CCFLAGS"] if not x in warn_flags]
-        self["CFLAGS"] = [x for x in self["CFLAGS"] if not x in warn_flags]
-        self["CXXFLAGS"] = [x for x in self["CXXFLAGS"] if not x in warn_flags]
+        self["CCFLAGS"] = [x for x in self["CCFLAGS"] if not (x.startswith("/W") or x.startswith("/w"))]
+        self["CFLAGS"] = [x for x in self["CFLAGS"] if not (x.startswith("/W") or x.startswith("/w"))]
+        self["CXXFLAGS"] = [x for x in self["CXXFLAGS"] if not (x.startswith("/W") or x.startswith("/w"))]
+        self.AppendUnique(CCFLAGS=["/w"])
     else:
-        self.Append(CCFLAGS=["-w"])
-        self.Append(CFLAGS=["-w"])
-        self.Append(CXXFLAGS=["-w"])
+        self.AppendUnique(CCFLAGS=["-w"])
 
 
 def add_module_version_string(self, s):