We set our own defaults for `CMAKE_CXX_FLAGS_${BUILD_TYPE}$ here: https://github.com/root-project/root/blob/master/cmake/modules/SetUpLinux.cmake#L97

Because cmake sets it own defaults for that variable (initialized by project(ROOT)) in the cache and without using set(... FORCE) our own settings are not picked up.

This is especially important because we would like to have RelWithDebInfo builds without NDEBUG, which is currently not possible (NDEBUG is part of the default cmake variable for CMAKE_CXX_FLAGS_RELWITHDEBINFO).