CI: Pass flag "-DOPTIONAL_WERROR=ON" to cmake

Author: ubruhin

.appveyor.yml

@@ -6,6 +6,6 @@ build_script:
   - git submodule update --init --recursive
   - mkdir build
   - cd build
-  - cmake ..
+  - cmake -DOPTIONAL_WERROR=ON ..
   - cmake --build .
   - C:\projects\optional\build\Debug\tests.exe

.travis.yml

@@ -254,4 +254,4 @@ install:
     - if [ "$CXX" = "clang++" ]; then export CXX="$COMPILER -stdlib=libc++"; fi
     - if [ "$CXX" = "g++" ]; then export CXX="$COMPILER"; fi
 
-script: mkdir build && cd build && cmake -DCXXSTD=$CXXSTD .. && make && ./tests
+script: mkdir build && cd build && cmake -DCXXSTD=$CXXSTD -DOPTIONAL_WERROR=ON .. && make && ./tests