We no longer use llvm-config to obtain various LLVM build directories
but instead use the variables set by find_package(LLVM). Also,
ensure that SOAAP LLVM's cmake files are being used by setting
LLVM_DIR when invoking cmake. So now:

cmake -G Ninja -DCMAKE_BUILD_TYPE=Debug -DLLVM_DIR=$HOME/path/to/llvm/build/share/llvm/cmake BUILD_SHARED_LIBS=OFF ..