windows: bundle GCC related DLLs

The following error occurred in Rroonga 4.0.2 x86-mingw32:

```
C:\Users\myokoym>ruby -e 'require "groonga"'
C:/Ruby193/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:36:in `require':
cannot load such file -- groonga.so (LoadError)
        from C:/Ruby193/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:36:i
n `require'
        from C:/Ruby193/lib/ruby/gems/1.9.1/gems/rroonga-4.0.2-x86-mingw32/lib/g
roonga.rb:45:in `rescue in <top (required)>'
        from C:/Ruby193/lib/ruby/gems/1.9.1/gems/rroonga-4.0.2-x86-mingw32/lib/g
roonga.rb:41:in `<top (required)>'
        from C:/Ruby193/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:60:i
n `require'
        from C:/Ruby193/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:60:i
n `rescue in require'
        from C:/Ruby193/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:35:i
n `require'
        from -e:1:in `<main>'
```

The cause is probably that `libwinpthread-1.dll` could not be found.

![depends-groonga so-rroonga-4 0 2-x86-mingw32](https://cloud.githubusercontent.com/assets/386687/3161386/3054776c-eb2b-11e3-96fa-5ec7c143c21e.png)