1. Grab LLVM sources:

  svn checkout http://llvm.org/svn/llvm-project/llvm/tags/RELEASE_30/final src

3. Run the magical configure.sh

  ./configure.sh

4. Build:

  cd build
  make -j

5. LLVM should have been built into the "build" subdirectory and
   installed in the "bin" subdirectory.