Resolve legacy_numeric_constants clippy lints

    warning: importing legacy numeric constants
      --> build.rs:43:5
       |
    43 | use std::u32;
       |     ^^^^^^^^
       |
       = help: remove this import
       = note: then `u32::<CONST>` will resolve to the respective associated constant
       = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#legacy_numeric_constants
       = note: `#[warn(clippy::legacy_numeric_constants)]` on by default

build.rs

@@ -40,7 +40,6 @@ use std::iter;
 use std::path::Path;
 use std::process::{self, Command, Stdio};
 use std::str;
-use std::u32;
 
 fn main() {
     let rustc = rustc_minor_version().unwrap_or(u32::MAX);