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
Ok, so just "cmake .." is not enough- due to the logic added above the LLVM_CONFIG variable must be specified. The developer has two options: 1.) issue "cmake -DLLVM_CONFIG=/path/to/llvm-config .." or 2.) issue "ccmake .." then press 'c' to configure, acknowledge the message, set LLVM_CONFIG in the GUI, then press 'c' and then 'g'.

Option (2) is the standard procedure for setting up a cmake build, but README.md might as well list option (1) for expediency.