This generates additional macros to help keeping implementation simple,
guarded with WIDL_using ifdefs, like this:

    #ifdef WIDL_using_Windows_Foo
    #define IFooVtbl __x_ABI_CWindows_CFoo_CIFooVtbl
    #define IFoo __x_ABI_CWindows_CFoo_CIFoo
    #define IFoo_DoFoo __x_ABI_CWindows_CFoo_CIFoo_DoFoo
    #endif /* WIDL_using_Windows_Foo */

Implementation files can define the desired WIDL_using preprocessor
macros before including the header, and then implement or use the
interface methods with the simple non-prefixed names instead.
Signed-off-by: Rémi Bernon <rbernon@codeweavers.com>
Signed-off-by: Jacek Caban <jacek@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>