add_custom_target(foobar
  COMMAND ${CMAKE_COMMAND} -E copy
    ${CMAKE_CURRENT_SOURCE_DIR}/generated.h.in
    ${CMAKE_CURRENT_BINARY_DIR}/generated.h
)

add_custom_command(
  OUTPUT generated.hpp
  # This test will fail if DEPENDS isn't accounted for in the codegen build graph
  DEPENDS foobar
  COMMAND ${CMAKE_COMMAND} -E copy ${CMAKE_CURRENT_BINARY_DIR}/generated.h
                                   ${CMAKE_CURRENT_BINARY_DIR}/generated.hpp
  CODEGEN
)

add_custom_target(hpp_creator ALL DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/generated.hpp)