Make the dependency on js_of_ocaml optional

We only build the rpclib.js sub-package (which can only be
used if js_of_ocaml is installed) only if js_of_ocaml is
installed.

This makes it easier for the xapi-project, since it doesn't
need to package js_of_ocaml.

Signed-off-by: David Scott <dave.scott@citrix.com>