cargo-build-system: cargo-build-flags: --cfg=unix in package recipe

[Danny Milosavljevic]

Hi,

so it seems I got the cargo-build-system to work for large-ish Rust dependency 
trees now. It still can't pick up transitive dependencies but hey :P

However, some of the Rust crates have a "windows" target which fails.

For these I'd like to pass "--cfg=unix" and I'd like to put it into the package 
recipe and not hardcode it into the build system.

I saw that there's a "cargo-build-flags" parameter. However, specifying it like

  (arguments
   `(#:cargo-build-flags '("--cfg=unix" "--release")))

gives me "invalid keyword argument"... 

Why?