Getting bullet to work on iOS is super easy. Ignore the instructions they give.
1) Create a new XCode iOS static library project and empty it out.
2) Add the entire bullet source tree.
3) You'll need to remove some CMake files manually, or XCode will complain about not knowing what they're for. There's also a few other things to skip like MiniCL, ibmsdk, etc.
4) Link it to your application like any XCode static library project.
I've never bothered with CMake to build bullet.