Correct appveyor.yml file.

Author: Tessil

appveyor.yml

@@ -5,25 +5,23 @@ environment:
           ARCH: Win32
           BOOST_LIBRARYDIR: C:\Libraries\boost_1_67_0\lib32-msvc-14.0
           CMAKE_GENERATOR: Visual Studio 14 2015
-          FLAGS: ""
 
         - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
           ARCH: x64
           BOOST_LIBRARYDIR: C:\Libraries\boost_1_67_0\lib64-msvc-14.0
           CMAKE_GENERATOR: Visual Studio 14 2015 Win64
-          FLAGS: ""
 
         - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
           ARCH: Win32
           BOOST_LIBRARYDIR: C:\Libraries\boost_1_67_0\lib32-msvc-14.1
           CMAKE_GENERATOR: Visual Studio 15 2017
-          FLAGS: "/permissive-"
+          CXXFLAGS: /permissive-
 
         - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
           ARCH: x64
           BOOST_LIBRARYDIR: C:\Libraries\boost_1_67_0\lib64-msvc-14.1
           CMAKE_GENERATOR: Visual Studio 15 2017 Win64
-          FLAGS: "/permissive-"
+          CXXFLAGS: /permissive-
 
 configuration:
     - Debug
@@ -33,7 +31,7 @@ build_script:
     - cd tests
     - mkdir build
     - cd build
-    - cmake .. -G"%CMAKE_GENERATOR%" -DCMAKE_CXX_FLAGS="%FLAGS%"
+    - cmake .. -G"%CMAKE_GENERATOR%"
     - cmake --build . --config %CONFIGURATION%
 
 test_script: