RUSTFLAGS handling

[gyakovlev]

https://github.com/gyakovlev/rust-dev-overlay/blob/5bde43d1f07f3cf55ab899ff394635a830ae182d/eclass/rust-crate.eclass#L147

it completely disregards user flags, leave it or make it append? prepend?

[kentfredric]

IME, it needs an auxilliary control to allow ebuilds to specify additional rust flags.

Then the final target should be run as:

RUSTFLAGS="<eclass flags> <ebuild flags> <user flags> "