Allow passing specific ocaml compiler flags for linking step only

[vkurchatkin]

It seems that at the moment ocaml-flags is passed for both compiling and linking, which makes using some flags impossible.

[bsansouci]

Which flags in particular are you referring to?

[vkurchatkin]

I'm interested in -output-obj and -output-complete-obj flags at the moment, but there are probably more flags like this.