Fix feature name

Co-Authored-By: gnzlbg <gnzlbg@users.noreply.github.com>

src/lib.rs

@@ -34,7 +34,7 @@
 #![no_std]
 #![cfg_attr(feature = "rustc-dep-of-std", no_core)]
 #![cfg_attr(target_os = "redox", feature(static_nobundle))]
-#![cfg_attr(feature = "const-extern-fn", feature(const_extern_fn))]
+#![cfg_attr(libc_const_extern_fn, feature(const_extern_fn))]
 
 #[macro_use]
 mod macros;

src/macros.rs

@@ -149,7 +149,7 @@ macro_rules! s_no_extra_traits {
 // 2. Allow users of this macro to mix 'pub fn foo' and 'pub const fn bar' within the same
 // 'f!' block
 cfg_if! {
-    if #[cfg(feature = "const-extern-fn")] {
+    if #[cfg(libc_const_extern_fn)] {
         #[allow(unused_macros)]
         macro_rules! f {
             ($(pub $({$constness:ident})* fn $i:ident(