cant compile

https://github.com/katzer/mruby-r3/commit/ba93e9e406cf559882650637339f4a76b527d605 introduced some regression, i get:

```
undefined method `defines' for #<struct MRuby::Gem::LinkerConfig libraries=["pthread"], library_paths=[], flags=[], flags_before_libraries=[], flags_after_libraries=[]>
rakefile:28:in `load'
```

when i try to include it into h2o