Fix cmake configuration

Hi, I have tried to fix the building using cmake. I tested this on a debian/unstable and it works. I am debian developer and I will try to upload Halide to debian. But first I would like to have a good build system configuration.

To build using cmake I am using::
    mkdir build
    cd build
    cmake -D LLVM_CONFIG=llvm-config-3.6 -D CLANG_PATH=clang-3.6 ..

Please take a look and tell me what you think
It is not necessary if you want to use the llvm installed in a "standard way" (like apt-get install llvm). But if you prefer specify this in the README.md I think that is a good idea.