Patch for compilation under windows x64 mingw

[Ramón García]

I have just noted that the just released 3.0.11 has this problem corrected.
Thanks.

2012/4/25 Ramón García <ramon.garcia.f@gmail.com>:
> Hello,
>
> Under Windows x64 it is not possible to build libffi as a shared
> library. In this platform, libtool needs the flag -no-undefined. This
> is done correctly under Windows x86.
>
> The fix is to add this flag under win64 like it is being done under win32.