ocamlbuild: fix plugin not building when no there are no targets
    
    An early-exit test in ocamlbuild was originally checked after plugin
    compilation and execution, but was moved before it by the
    modular-plugins changes. This had the effect of exiting early as soon
    as they were no targets, even if -just-plugin was passed.
    
    git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.01@14030 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02