I know this will work, but I feel a bit uncomfortable defining `APEX_SOURCE_DIR` here. It's actually defined by APEX itself as well and could in principle change once you do `add_subdirectory(${APEX_SOURCE_DIR} ...)`. Maybe you can just do  ```  add_subdirectory(${APEX_ROOT}/src/apex ...)  include_directories(${APEX_SOURCE_DIR})  ```  ?