The variable CMAKE_CXX_FLAGS does not contain the values stored by the property set by add_compile_options. You need to modify the property for the targets created downstream in Prototypes/dpdk_test_static_library/dpdk/CMakeLists.txt. After the target is created then you can modify COMPILE_OPTIONS property for that target. Or you can try to modify the directory property COMPILE_OPTIONS before calling add_subdirectory().