There's a few DLL's floating around for libffi, however none of them seem to have the symbol "ffi_prep_cif_var" which seems to be needed for variadic functions.
I can version variadic support out and compile your bindings, the tests will work fine on win32. I've also tried compiling libffi on my own but had no success. I can make a pull with the import lib, the DLL (it's only 99KB), and the versioned out sections, if you'd like that.
