```caller_dir``` should be the path where build_config.rb is located. The idea is that things like this are possible:

```
MRuby::Build.new do |conf|
  toolchain :gcc
  conf.gem 'examples/mrbgems/ruby_extension_example'

  ...
```

This is integrating a gem from the example folder in case the build_config.rb is used from the MRUBY_ROOT. In case you are using a custom build_config.rb in another folder you can keep your GEMs separate from the MRUBY_ROOT but relative from your build_config.rb. This is from a time where we didn't had gemboxes yet.