duplicate arguments in js_of_eliom

[agarwal]

js_of_eliom takes some arguments specific to it, plus all js_of_ocaml args, plus all ocamlc args. The final result has these flags duplicated: version, I, o, linkall. Similarly, the -ppx option is duplicated in eliomc (as reported in #273). All this causes confusion, and I wonder if argument parsing could be implemented more robustly, so that such bugs are impossible.

FYI, I'm discovering these issues due to my new API for constructing ocamlbuild Command.t values.