[OCaml] De-duplicate llvm_raise and llvm_string_of_message.
[OCaml] [autoconf] Migrate to ocamlfind.

This commit updates the OCaml bindings and tests to use ocamlfind.
The bindings are migrated in order to use ctypes, which are now
required for MCJIT-backed Llvm_executionengine.
The tests are migrated in order to use OUnit and to verify that
the distributed META.llvm allows to build working executables.

Every OCaml toolchain invocation is now chained through ocamlfind,
which (in theory) allows to cross-compile the OCaml bindings.

The configure script now checks for ctypes (>= 0.2.3) and
OUnit (>= 2). The code depending on these libraries will be added
later. The configure script does not check the package versions
in order to keep changes less invasive.

Additionally, OCaml bindings will now be automatically enabled
if ocamlfind is detected on the system, rather than ocamlc, as it
was before.
[OCaml] Enable backtraces in tests.
[OCaml] Test code emission in Llvm_target.

Prior to this commit, the Llvm_target tests (ab)used
the Llvm_executionengine as a mechanism to initialize at least some
target. This needlessly restricted tests to builds which can emit
code for their host architecture.
[OCaml] Expose LLVM{Get,Set}DLLStorageClass.
[OCaml] Expose LLVMCloneModule.