Pass -pa-ounit-drop to pa_ounit if --disable-tests

Relates to #13.

Pass `-pa-ounit-drop` option to pa_ounit if configured with
`--disable-tests`.  This change is hacky because
`config/ocamlbuildflags.sh` forcely overwrites `ocamlbuildflags` flag
in order to tell `drop_test` tag to `myocamlbuild.ml` indirectly.