Use `#[link(cfg(..))]` in preparation for libstd
In preparation for rust-lang/rust#37545 this is adding the appropriate
directives to libc to get included.
diff --git a/src/lib.rs b/src/lib.rs
index dcc4791..2bc26aa 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -75,6 +75,7 @@
// Attributes needed when building as part of the standard library
#![cfg_attr(stdbuild, feature(no_std, core, core_slice_ext, staged_api, custom_attribute, cfg_target_vendor))]
+#![cfg_attr(stdbuild, feature(link_cfg))]
#![cfg_attr(stdbuild, no_std)]
#![cfg_attr(stdbuild, staged_api)]
#![cfg_attr(stdbuild, allow(warnings))]