Fix failing check for ocamlc existence

[ulugbekna]

The extension checks for existence of ocamlc binary for some reason related to checking whether the current sandbox contains ocaml installation. This doesn't work in local opam switches

$ /usr/local/bin/opam exec --switch=/Users/ulugbekna/code/merlin --set-switch --root /Users/ulugbekna/.opam -- ocamlc --version
[ERROR] Command not found 'ocamlc'

which results in a user-facing error notification on extension startup.

AFAIU, opam exec on a local switch checks for commands in <local-switch-path>/bin/, which doesn't contain ocamlc (new switch creation was suspiciously fast, is there a new hack in opam that allows to avoid copying ocamlc?)

[rgrinberg]

Not quite sure I understand this one. How can the local switch not contain ocamlc? Is it because the local switch is using the system OCaml? In any case, I would expect the command /usr/local/bin/opam exec --switch=/Users/ulugbekna/code/merlin --set-switch --root /Users/ulugbekna/.opam -- ocamlc --version to pick up the system ocamlc from $PATH.