Auto merge of #3356 - ibabushkin:feature-build-rustflags, r=alexcrichton

Implemented string lookup for `build.rustflags` config key

This addresses the immediate issue described in #3052 .
I am, however, unsure about the current state of the deeper issues mentioned in that issue, but if needed, I can take stab at them as well. :)

diff --cc src/cargo/ops/cargo_rustc/context.rs
Simple merge