Adding some conditionals in the language

This is similar to #819 but with a somewhat broader scope. The idea would be to support (if <cond> <then> [else]) at the toplevel of a jbuild.

This would allow encoding the example of #819, which was:

(alias
 ((name runtest)
  (deps (foo.exe))
  (action (run ${<}))
  (enabled_if (and (<> ${os} win32) (>= ${ocaml_version} 4.0.5)))))

as

(if (and (<> ${os} win32) (>= ${ocaml_version} 4.0.5))
    (alias
     ((name runtest)
      (deps (foo.exe))
      (action (run ${<})))))

(of course I have nothing with the sugar proposed in #819).

But would also allow doing things like:

(if (= ${ocaml_version} "4.07._")
    (copy_files# 407/*.ml{,i}))
(if (= ${ocaml_version} "4.06._")
    (copy_files# 406/*.ml{,i}))
...

which are needed for merlin and currently require us to use ocaml jbuilds.

---

I'm only proposing to support this at the toplevel, but this could be pushed down to arbitrary places (after all, (select ...) already does similar things).