_ffi and Windows stdcall symbols - clarification

[Mike Thomas]

Actually:

| Windows stdcall library entry points may be a problem as shown
| below.  Presumably either a different specifier is needed or the
| syntax of _ffi should be extended - possibly to include a header
| file to obtain a prototype.

The point about the possibility of specifying a header is meant to be a
completely separate suggestion for the case where MLton compiles via C with
gcc as opposed to the native code generator, rather than a possible solution
to the addition of the stdcall calling convention to the already existing
ccall default for _ffi.