fix: Ensure that const extern fn is always enabled

In [1] this conditional was dropped in favor of a Cargo feature, which
was turned on by default in [2]. However, this did not help the case
where `--no-default-features` is passed.

Unfortunately we still can't drop this config entirely since `ctest`
cannot parse the syntax, so change back to useing a `cfg` to control
constness rather than a Cargo feature.

Fixes: rust-lang#4149

[1]: rust-lang#4105
[2]: rust-lang#4134

build.rs

@@ -13,6 +13,8 @@ const ALLOWED_CFGS: &'static [&'static str] = &[
     "freebsd13",
     "freebsd14",
     "freebsd15",
+    // FIXME(ctest): this config shouldn't be needed but ctest can't parse `const extern fn`
+    "libc_const_extern_fn",
     "libc_deny_warnings",
     "libc_ctest",
 ];
@@ -74,6 +76,9 @@ fn main() {
         set_cfg("libc_deny_warnings");
     }
 
+    // Set unconditionally when ctest is not being invoked.
+    set_cfg("libc_const_extern_fn");
+
     // check-cfg is a nightly cargo/rustc feature to warn when unknown cfgs are used across the
     // codebase. libc can configure it if the appropriate environment variable is passed. Since
     // rust-lang/rust enforces it, this is useful when using a custom libc fork there.

src/macros.rs

@@ -199,7 +199,7 @@ macro_rules! e {
 // cfg completely.
 // FIXME(ctest): ctest can't handle `$(,)?` so we use `$(,)*` which isn't quite correct.
 cfg_if! {
-    if #[cfg(feature = "const-extern-fn")] {
+    if #[cfg(libc_const_extern_fn)] {
         /// Define an `unsafe` function that is const as long as `const-extern-fn` is enabled.
         macro_rules! f {
             ($(