VMS Help DCE_IDL, DCL Command Interface, QUALIFIERS, /PREPROCESS  *Conan The Librarian (sorry for the slow response - running on an old VAX)

      /PREPROCESS
      /NOPREPROCESS (default)

        These qualifiers are equivalent to the
        -cpp_cmd 'c_preprocessor_command_line' and -no_cpp
        arguments in the universal syntax.