To make Human Plus work, the first thing you need to do is to download LLVM 5.x source from their site, or clone their git repository, (or the mirror on GitHub).
Build LLVM with CMake in a directory, and use your build system to compile the entire project. Please note that LLVM is required for the Human Plus Compiler. You can avoid this project if this is not in your intentions.
Now you can build Human Plus with CMake by passing the directory containing the LLVM build (not the source) with the LLVM_BUILD_DIR variable.