Define OpenMW specific C++ flags

CI/before_script.linux.sh

@@ -32,6 +32,7 @@ declare -a CMAKE_CONF_OPTS=(
     -DCMAKE_INSTALL_PREFIX=install
     -DCMAKE_C_FLAGS='-Werror'
     -DCMAKE_CXX_FLAGS="${CXX_FLAGS}"
+    -DOPENMW_CXX_FLAGS="-Werror=implicit-fallthrough"
 )
 
 if [[ $CI_OPENMW_USE_STATIC_DEPS ]]; then

CMakeLists.txt

@@ -543,8 +543,8 @@ if (BUILD_OPENCS)
     add_subdirectory (extern/osgQt)
 endif()
 
-if (CMAKE_CXX_COMPILER_ID STREQUAL GNU OR CMAKE_CXX_COMPILER_ID STREQUAL Clang)
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Werror=implicit-fallthrough")
+if (OPENMW_CXX_FLAGS)
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${OPENMW_CXX_FLAGS}")
 endif()
 
 # Components