def cpp_files_libraries(aux_libraries)
      arduino_library_src_dirs(aux_libraries).map { |d| cpp_files_in(d) }.flatten.uniq
    end
def cpp_files_libraries(aux_libraries)
      arduino_library_src_dirs(aux_libraries).map { |d| cpp_files_in(d) }.flatten.uniq
    end
CPP files that are part of the 3rd-party libraries we're including
 @param [Array<String>] aux_libraries
 @return [Array<Pathname>]