change from unstable cargo feature to RUSTFLAGS

[nikomatsakis]

So we encountered some problems that people are relying on things gated by "feature=unstable" (e.g., #364). This isn't really surprising. I was thinking that a better approach would be use a cfg flag that is not tied to a feature. Therefore, in order to build Rayon with experimental features, you would have to do:

RUSTFLAGS='--cfg rayon_unstable' cargo build

This flag will propagate to the rayon create, which will then include unstable features. This has the same "infectious" property that Rust nightly builds do: you can't use an unstable feature in your crate without the top-level crate being forced to acknowledge it.

[nikomatsakis]

I implemented this idea idea in #368

[sanmai-NL]

@nikomatsakis: I read your issue report and the issue you referred to, but I don’t yet understand what the advantage of using a cfg flag is here, over using a Cargo feature. I suppose you implicitly were talking about keeping the rayon-core and rayon in sync. Before I continue, is that correct?