Provide option to restrict RUSTFLAGS to non-build-dependencies

[Manishearth]

Currently, setting RUSTFLAGS will compile build dependencies with the provided options as well. This means that using things like the sanitizer support may not work since the build dependencies will be compiled with sanitizers on as well, and they may fail.

This probably also means that RUSTFLAGS isn't very useful when cross compiling.

(This is making it hard to write nice wrappers for fuzzing)

[alexcrichton]

As a hack for now you can pass RUSTFLAGS and --target $host_target to get this working

[RalfJung]

I am right now having the opposite problem... I am setting RUSTFLAGS, and it does not get set for build dependencies, leaving me unable to tweak the flags they are compiled with. Has something changed here?

EDIT: Ah, I am using xargo. So probably it passes --target. So now I need a way to set RUSTFLAGS despite cross-compiling...

[lu-zero]

I stumbled upon this trying to building with -Z sanitize, is it possible to make progress on that?