Fix conflict between no_std and no_core when building rustc with custom libc

Author: IsaacWoods

src/lib.rs

@@ -25,7 +25,7 @@
 // Enable extra lints:
 #![cfg_attr(feature = "extra_traits", deny(missing_debug_implementations))]
 #![deny(missing_copy_implementations, safe_packed_borrows)]
-#![no_std]
+#![cfg_attr(not(feature = "rustc-dep-of-std"), no_std)]
 #![cfg_attr(feature = "rustc-dep-of-std", no_core)]
 #![cfg_attr(
     any(feature = "rustc-dep-of-std", target_os = "redox"),