Use the Cargo.toml version of the check-cfg squelching

It doesn't make older version of cargo complain.

https://doc.rust-lang.org/nightly/rustc/check-cfg/cargo-specifics.html#check-cfg-in-lintsrust-table

Fixes #576

Cargo.toml

@@ -83,3 +83,7 @@ required-features = ["static-curl", "ssl"]
 name = "multi-dl"
 path = "examples/multi-dl.rs"
 required-features = ["ssl"]
+
+[lints.rust.unexpected_cfgs]
+level = "warn"
+check-cfg = ["cfg(need_openssl_init, need_openssl_probe)"]

build.rs

@@ -1,12 +1,6 @@
 use std::env;
 
 fn main() {
-    println!(
-        "cargo::rustc-check-cfg=cfg(\
-            need_openssl_init,\
-            need_openssl_probe,\
-        )"
-    );
     // OpenSSL >= 1.1.0 can be initialized concurrently and is initialized correctly by libcurl.
     // <= 1.0.2 need locking callbacks, which are provided by openssl_sys::init().
     let use_openssl = match env::var("DEP_OPENSSL_VERSION_NUMBER") {