appveyor.yml: Add MSVC 16 2019 support

appveyor.yml

@@ -16,6 +16,13 @@ environment:
       LINKER_FLAGS: ""
       GENERATOR: Visual Studio 15 2017
 
+    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
+      configuration: Debug
+      platform: x86
+      CXX_FLAGS: ""
+      LINKER_FLAGS: ""
+      GENERATOR: Visual Studio 16 2019
+
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
       configuration: Debug
       COMPILER: mingw
@@ -50,16 +57,16 @@ environment:
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
       configuration: Release
       platform: x86
-      CXX_FLAGS: ""
+      CXX_FLAGS: "/permissive- /std:c++latest /utf-8"
       LINKER_FLAGS: ""
       GENERATOR: Visual Studio 15 2017
 
-    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
+    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
       configuration: Release
       platform: x86
-      CXX_FLAGS: "/permissive- /std:c++latest /utf-8"
+      CXX_FLAGS: ""
       LINKER_FLAGS: ""
-      GENERATOR: Visual Studio 15 2017
+      GENERATOR: Visual Studio 16 2019
 
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
       configuration: Release
@@ -71,16 +78,16 @@ environment:
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
       configuration: Release
       platform: x64
-      CXX_FLAGS: ""
-      LINKER_FLAGS: ""
+      CXX_FLAGS: "/permissive- /std:c++latest /utf-8 /F4000000"
+      LINKER_FLAGS: "/STACK:4000000"
       GENERATOR: Visual Studio 15 2017
 
-    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
+    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
       configuration: Release
       platform: x64
-      CXX_FLAGS: "/permissive- /std:c++latest /utf-8 /F4000000"
-      LINKER_FLAGS: "/STACK:4000000"
-      GENERATOR: Visual Studio 15 2017
+      CXX_FLAGS: ""
+      LINKER_FLAGS: ""
+      GENERATOR: Visual Studio 16 2019
 
 init:
   - cmake --version