macro (add_sources)
  file (RELATIVE_PATH _relPath "${CMAKE_SOURCE_DIR}" "${CMAKE_CURRENT_SOURCE_DIR}")
  foreach (_src ${ARGN})
    if (_relPath)
      list (APPEND SRCS "${_relPath}/${_src}")
    else()
      list (APPEND SRCS "${_src}")
    endif()
  endforeach()
  if (_relPath)
    # propagate SRCS to parent directory
    set (SRCS ${SRCS} PARENT_SCOPE)
  endif()
endmacro()
