Fixes to build on Linux (64-bit Arch Linux)

This compiles and runs successfully on my machine. I'm not sure how to make it more portable - you can see I hard-coded the `/usr/lib32` path which I think only exists on an amd64 multilib setup.

This will also statically link in Box2D from the include folder, since I didn't have a package for a 32-bit version of the library. But based on the README comment about Box2D combining src and include, maybe static linking is the norm for this library?