(sorry for my english
i think i should build an "universal" binary. this runs on both platforms. but at the moment i don't know which build i used
you can checkout the source of the gem in the "Contrib"-directory of Box2D. Then you can try to build the gem.
- first you should change the XCode-Project (Box2d-Lib) to a static library.
- check the build for static stl-compile/link.
- after you have builded it, you should copy the libbox2d.a to the "Library"-directory
- you can call ruby extconf.rb, make
- ruby ruby.gemspec
- then you have the gem.