-cpp_opt 'command_options'

   Specifies additional options to be passed to the C preprocessor.
   You can add options to the command line used to invoke the C
   preprocessor independent of the -cpp_cmd argument.  The IDL compiler
   concatenates the -cpp_cmd, -cpp_opt, -D, -U, -I arguments and the
   source filename into a command used to invoke the C preprocessor.
   The compiler repeats this process for each Attribute Configuration
   File (ACF) and IDL file.