Allow warnings during a libstd build

Not gonna gain much from issuing warnings about various aspects! This is managed
separately from the standard library

Author: alexcrichton

src/lib.rs

@@ -62,6 +62,7 @@
 #![cfg_attr(stdbuild, feature(no_std, core, core_slice_ext, staged_api))]
 #![cfg_attr(stdbuild, no_std)]
 #![cfg_attr(stdbuild, staged_api)]
+#![cfg_attr(stdbuild, allow(warnings))]
 #![cfg_attr(stdbuild, unstable(feature = "libc",
                                reason = "use `libc` from crates.io",
                                issue = "27783"))]