Detect availability and use `CARGO_CFG_FEATURE`

[epage]

Cargo normalizes CARGO_FEATURE_* feature names, making it so there isn't an exact 1:1 between the env variable and Cargo.toml (see rust-lang/cargo#3702).

As of Cargo 1.85, Cargo now provides CARGO_CFG_FEATURE, see rust-lang/cargo#14902.