Don't rely on find_package to find LLVM.

find_package only works if LLVM was built with cmake and this isn't the
case with LLVM binaries shipped with various Linux distributions (e.g.
Ubuntu). This revives the FindLLVM cmake module that was previously
removed in commit b4665b72f261f78ebfc14abae66f60f178ed400b.
CMake 2.8.12 is required in order to use PUBLIC in target_link_libraries().
Add llvm-3.3-dev to the list of packages to install.
Remove unnecessary add-apt-repository.