The generation of the library interface supplied by this function only
works within the same directory as that the target was defined. By
adding a custom target named GENERATE_<TARGET> now also interface files
will be generated by making a target dependend on the generate target.

Example:
/CMakeLists.txt
  set(MY_INCL_SRC ${CMAKE_CURRENT_SOURCE_DIR}/fbs/my_incl.fbs)
  flatbuffers_generate_headers(TARGET my_incl
                               SCHEMAS ${MY_INCL_SRC})

  add_subdirectory(app)

/app/CMakeLists.txt
  add_executable(app src/test.cpp)
  target_link_libraries(app my_incl)
  add_dependencies(app GENERATE_my_incl)