Revert "SCons: Add CPPEXTPATH for external includes"

modules/minimp3/SCsub

@@ -8,7 +8,11 @@ env_minimp3 = env_modules.Clone()
 
 thirdparty_dir = "#thirdparty/minimp3/"
 
-env_minimp3.Prepend(CPPEXTPATH=[thirdparty_dir])
+# Treat minimp3 headers as system headers to avoid raising warnings. Not supported on MSVC.
+if not env.msvc:
+    env_minimp3.Append(CPPFLAGS=["-isystem", Dir(thirdparty_dir).path])
+else:
+    env_minimp3.Prepend(CPPPATH=[thirdparty_dir])
 
 if not env["minimp3_extra_formats"]:
     env_minimp3.Append(CPPDEFINES=["MINIMP3_ONLY_MP3"])