[plt-scheme] stdcall convention and FFI

Hi there,

I am trying to pass a Scheme callback to EnumWindows, a Win32 call:

BOOL EnumWindows(          WNDENUMPROC lpEnumFunc,
    LPARAM lParam
);

The lpEnumFunc parameter is a callback with a type like this:

BOOL CALLBACK EnumWindowsProc(          HWND hwnd,
    LPARAM lParam
);

The CALLBACK qualifier above indicates that the stdcall convention is
used. I did not find a way to declare this to the FFI.  Did I miss
something?

Thanks in advance,
-pp