/sys$common/syshlp/helplib.hlb DCE_IDL, ARGUMENTS, -cpp_cmd  *Conan The Librarian (sorry for the slow response - running on an old VAX)

   -cpp_cmd 'c_preprocessor_command_line'

   Allows you to specify a C preprocessor other than the default.  The
   compiler invokes the C preprocessor found in that command line.  The
   output of the C preprocessor is an expanded version of the input
   file(s) containing replacement text for any preprocessor directives
   (for example, the #include preprocessor directive).