We note that, as of version 3.12 of CMake, OBJECT libraries are another viable approach to organizing large projects. The only modification to our example would be in the CMakeLists.txt for the libraries. The sources would be compiled into object files: neither archived into a static archive, nor linked into a shared library. For example:

add_library(io OBJECT "")

target_sources(io
  PRIVATE
    io.cpp
  PUBLIC
    ${CMAKE_CURRENT_LIST_DIR}/io.hpp
  )

target_include_directories(io
  PUBLIC
    ${CMAKE_CURRENT_LIST_DIR}
  )

The top-level CMakeLists.txt remains unchanged: the automata executable target links these object files into the final executable. Usage requirements, such as include directories, compile flags, and link libraries set on the OBJECT libraries will correctly be inherited. For more details on this new feature of OBJECT libraries introduced in CMake 3.12 refer to the official documentation: https://cmake.org/cmake/help/v3.12/manual/cmake-buildsystem.7.html#object-libraries