Suppress unknown_lints warning on old rustc

    warning: unknown lint: `unexpected_cfgs`
      --> build.rs:37:10
       |
    37 | #![allow(unexpected_cfgs)]
       |          ^^^^^^^^^^^^^^^
       |
       = note: `#[warn(unknown_lints)]` on by default

build.rs

@@ -34,6 +34,7 @@
 //     macro API and catching a panic if it isn't available. Enabled on Rust
 //     1.57+.
 
+#![allow(unknown_lints)]
 #![allow(unexpected_cfgs)]
 
 use std::env;