An IDL specification may contain one or more import,
importlib or #include declarations, instructing
the compiler to slurp in information from some other file.
The compiler looks for the file to include along a builtin list
of directories (notably including .).
-i<dirs> or --include-dir <dirs>
This option prepends a colon separated list of dirs to
the default import search path.
-I<dirs>
In case you're initially washing the IDL specification by running
it through the CPP (i.e., using the -cpp option), the
-I option prepends the list of include search path
CPP will look in when it encounters a #include statement.