[PATCH] Makefile improvements for the ocaml binding

[Jacques-Pascal Deplaix]

Hi,

I've done two patches to improve the build system for the ocaml binding.

The first one [1] is basically adding a rule that just build the 
binding. The default rule builds the documentation and perhaps you don't 
want that (in most cases, you don't).

The second one [2] allows to compile the binding without compiling llvm 
itself. Instead, we use the system installed llvm. So I introduced a 
variable that explicitly ask for the llvm-config you want.

With those two patches you can now just do:
$ make build SYSTEM_LLVM_CONFIG="llvm-config-3.5"

after having executing the configure script and you are on your way for 
a <1 minute compilation instead of the >2 hours as before when we needed 
to compile llvm itself.

Tested successfully for me and the opam users: 
https://github.com/ocaml/opam-repository/pull/1942

[1]: 
https://github.com/jpdeplaix/llvm/commit/5835680737ab169841aa2105ed7558aa2895265d
[2]: 
https://github.com/jpdeplaix/llvm/commit/c653302fa2572f4711b458eafc969b186c64da3d

Cheers,