gmp 5.1.1 tests failures on 64 bin windows with AMD processors

[Darin Ohashi]

>   Building gmp 5.1.1 using
>   
>   ./configure --host=x86_64-w64-mingw32
>   
> That's a quite bogus configuration, basically disabling CPU
> optimisation.  This sent me chasing the bug in the wrong direction,
> since I assumed you where using a bulldozer config here.

Is there a better way to tell configure to look for the
x86_64-w64-mingw32 tools?  Without that host, configure uses the cygwin
tools, which I don't want.  If I specify --host=bulldozer-w64-mingw32,
then configure looks for a compiler called bulldozer-w64-mingw32, which
doesn't exists so it falls back to cygwin gcc.

Darin