Fixed: build error on Windows Ruby x64 (with ImageMagick 6.8.0-10 or Ima...

Hi. Nice to see it maintained again!

Fixed: build error on Windows Ruby x64 (with ImageMagick 6.8.0-10 / ImageMagick 6.8.7-7)
  ruby 2.0.0p451 (2014-02-24) [x64-mingw32]

```
  if RUBY_PLATFORM =~ /x64/
    $LOCAL_LIBS = '-lCORE_RL_magick_'
  else
    $LOCAL_LIBS = '-lCORE_RL_magick_ -lX11'
  end
```


> For the 64-bit build, you will also need to disable X11 support.

<cite>[ImageMagick: Advanced Windows Source Installation](http://www.imagemagick.org/script/advanced-windows-installation.php)</cite>