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).

Start by reading the referenced issue #819 and the existing top-level jbuild handling, then compare it with the mentioned (select ...) form. The work is complete when top-level (if <cond> <then> [else]) supports the shown alias and version-specific copy_files# examples, with behavior defined for the proposed scope.