[OCaml] fix tests in the CMake static library codepath

Summary:
OCaml bindings built statically with CMake are good for nothing, as
shown by the test failures. Make two changes to make tests pass:

1. Fix AddOCaml CMake rule to link in the case of static lib.
2. Add the LIBDIR variable required by lit.cfg.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>

Reviewers: whitequark, rafael

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D6806