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
Are all the files committed for this PR? There are no changes listed for src/CMakeLists.txt, but this file uses variables removed from above like LLVM_BIN to call llvm-config and obtain libraries to link with.  When I build with the branch, libHalide.dylib fails to link due to missing symbols.