Constant SYSTEM_PATH_ENV_VAR 

pub const SYSTEM_PATH_ENV_VAR: &str = "CINRS_SYSTEM_INCLUDE_PATH";

The environment variable that replaces system_directories’s per-target default, split the way the platform splits PATH.

This is the only way to name the directories on a target whose default cinrs does not know — Apple’s, whose SDK path is knowable only from xcrun --show-sdk-path, and Windows — and the only way to point a cross build at a sysroot.