appveyor: Treat warnings as errors

appveyor.yml

@@ -29,8 +29,8 @@ install:
 
 build_script:
 - cd c:\projects\assimp
-- if "%platform%" equ "x64" (cmake CMakeLists.txt -G "Visual Studio %Configuration% Win64")
-- if "%platform%" equ "x86" (cmake CMakeLists.txt -G "Visual Studio %Configuration%")
+- if "%platform%" equ "x64" (cmake CMakeLists.txt -DASSIMP_WERROR=ON -G "Visual Studio %Configuration% Win64")
+- if "%platform%" equ "x86" (cmake CMakeLists.txt -DASSIMP_WERROR=ON -G "Visual Studio %Configuration%")
 - if "%platform%" equ "x64" (msbuild /m /p:Configuration=Release /p:Platform="x64" Assimp.sln)
 - if "%platform%" equ "x86" (msbuild /m /p:Configuration=Release /p:Platform="Win32" Assimp.sln)