Fix warnings on new Rust

Author: YaLTeR

Cargo.toml

@@ -5,6 +5,7 @@ authors = ["Ivan Molodetskikh <[email protected]>"]
 description = "A circular buffer-like queue."
 license = "MIT/Apache-2.0"
 build = "build.rs"
+edition = "2015"
 
 readme = "README.md"
 documentation = "https://docs.rs/circular-queue"

build.rs

@@ -1,6 +1,7 @@
 extern crate version_check;
 
 fn main() {
+    println!("cargo:rustc-check-cfg=cfg(has_extern_crate_alloc)");
     if version_check::is_min_version("1.36.0").unwrap_or(false) {
         println!("cargo:rustc-cfg=has_extern_crate_alloc");
     }