How to set RUSTFLAGS in a maturin project?

[yurivict]

This pull requests makes maturin to respect rustflags in the Rust's config file.
However, adding

[build]
rustflags = ["…", "…"]

like is documented here doesn't seem to work.

What is the correct way to add RUSTFLAGS?

Can this be done through the arguments or environment of the Python's build project, or do config files need to be modified?