Get information about the build from environment variables
let mut be = build_env::BuildEnv::from_env().unwrap();
let _: Option<OsString> = be.var_os("CC");
Prints all used env vars in cargo dependency format:
cargo:rerun-if-env-changed=CC_target-triple
cargo:rerun-if-env-changed=CC_target_triple
cargo:rerun-if-env-changed=HOST_CC
cargo:rerun-if-env-changed=CC
