Feature flag support

[gnzlbg]

To test libc we need to also support building a crate with --features and with --no-default-features.

[gnzlbg]

Or are --features and --no-default-features already read by cargo ? The cargo::utils::Config takes an env variable, which might result in the args being read.

[ibabushkin]

Well, the issue is that we currently cannot really pass anything to cargo's CLI frontend from our code, as the implementation is private and doesn't directly generate a Config afaict. I'm also not sure how passing environment to the config would help with forcing cargo to honour flags we have no way of passing atm. Am I missing something?

However, we have functionality to make cargo not build default features (which you implemented :). It is subtly broken in some cases though, for instance if one has dependencies between targets and features (and we try building all targets unconditionally)

[gnzlbg]

cc @ehuss

[ibabushkin]

To summarize the implementation: we basically can pass --features and --all-features over the same channel as --no-default-features to cargo's CompileOptions, which I seem to have missed previously.