wip debug appveyor

Author: Théo Delrieu

.github/external_ci/appveyor.yml

@@ -48,7 +48,7 @@ environment:
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
       configuration: Release
       platform: x86
-      CXX_FLAGS: "/W4 /WX"
+      CXX_FLAGS: "/permissive- /std:c++latest /utf-8 /W4 /WX"
       CMAKE_OPTIONS: "-DJSON_ImplicitConversions=OFF"
       GENERATOR: Visual Studio 16 2019