code: def add_external_library(library):
      
  
      for command in ['build', 'build_ext', 'install']:
          add_command_option(command, str('use-system-' + library),
                             'Use the system {0} library'.format(library),
                             is_bool=True)
docs: Add a build option for selecting the internal or system copy of a library.
  
      Parameters
      ----------
      library : str
          The name of the library.  If the library is `foo`, the build
          option will be called `--use-system-foo`.