Problem in building swift

I was build swift on Mac OS and got error "llvm/utils/llvm-build/llvm-build: [Error 2] no such file or directory ". Then I cloned utils folder from llvm (Github).

Now, I am getting an error "llvm-build: error: unable to infer LLVM source root, please specify".

You should checkout the dependencies first.

git clone
./swift/utils/update-checkout --clone

I have already done that. I followed every instruction on README.txt but unable to build it. I don't know why I am getting an error.