do hipe compilation only if configured in app env

This pull request makes HiPE compilation an opt-in. To compile Jazz with HiPE, one must include `jazz: [native: true]` in `config.exs` of the depending project (the one that uses Jazz). 

If you don't like this approach, ping me on IRC to discuss whether something else might work. In any case, I don't think HiPE should be used by default.