-Idirectory

   Specifies a directory name that contains imported interface
   definition files.  You can specify more than one directory by
   specifying additional -Idirectory arguments on the command line.
   The compiler searches the directories in the order you list them.
   If a file is present in more than one directory, the compiler takes
   the first occurrence of the file.  The default behavior of the
   compiler is to first search the current directory, then all
   directories you specify, then the system IDL directory.  The
   directory you specify is also passed to the C preprocessor and the
   C compiler.