Install LLVM OCaml bindings (and UTop)

I put on a Stackoverflow answer here

1. Build LLVM with configuration: --with-ocaml-libdir =ocamlc -where``

2. To compile LLVM codes: ocamlc -cc g++ llvm.cma test.ml -o test

3. To make UTop work:

   • create a myutop_main.ml file with: let () = UTop_main.main ()

   • create a custom llvmutop: ocamlfind ocamlmktop -o llvmutop -thread -linkpkg -package utop llvm.cma myutop_main.ml -cc g++

4. If you just want OCaml's build-in toplevel:

   • ocamlmktop -o llvmtop llvm.cma -cc g++