Deny warnings in build system and in rustc_codegen_gcc

Author: GuillaumeGomez

build_system/src/main.rs

@@ -1,3 +1,5 @@
+#![deny(warnings)]
+
 use std::{env, process};
 
 mod build;

src/lib.rs

@@ -23,6 +23,7 @@
 #![recursion_limit = "256"]
 #![warn(rust_2018_idioms)]
 #![warn(unused_lifetimes)]
+#![deny(warnings)]
 #![deny(clippy::pattern_type_mismatch)]
 #![allow(clippy::needless_lifetimes, clippy::uninlined_format_args)]