message: meson: add multiple headers target
diff: project('nlohmann_json', 'cpp')
  -nlohmann_json_inc = include_directories('single_include')
  -
  nlohmann_json_dep = declare_dependency(
  - include_directories : nlohmann_json_inc
  + include_directories: include_directories('single_include')
  +)
  +
  +nlohmann_json_multiple_headers = declare_dependency(
  + include_directories: include_directories('include')
  )
  