First build LLVM: http://clang.llvm.org/get_started.html
Then build BRL-CAD: http://brlcad.org/wiki/Compiling

but run CMake using

`CC=<path_to_clang_bin> CXX=<path_to_clang++_bin> cmake .. -DBRLCAD_ENABLE_STRICT=OFF -DBRLCAD_BUNDLED_LIBS=ON`