Rollup merge of #108898 - ferrocene:pa-libc-check-cfg, r=Mark-Simulacrum

Set `LIBC_CHECK_CFG=1` when building Rust code in bootstrap

Downstream forks of the Rust compiler might want to use a custom `libc` to add support for targets that are not yet available upstream. Adding a patch to replace `libc` with a custom one would cause compilation errors though, because Cargo would interpret the custom `libc` as part of the workspace, and apply the check-cfg lints on it.

Since rust-lang/libc#3037, the `libc` build script emits check-cfg flags only when the `LIBC_CHECK_CFG` environment variable is set, so this PR allows the use of custom `libc`s.

src/bootstrap/builder.rs

@@ -1721,6 +1721,15 @@ impl<'a> Builder<'a> {
 
         cargo.env("RUSTC_VERBOSE", self.verbosity.to_string());
 
+        // Downstream forks of the Rust compiler might want to use a custom libc to add support for
+        // targets that are not yet available upstream. Adding a patch to replace libc with a
+        // custom one would cause compilation errors though, because Cargo would interpret the
+        // custom libc as part of the workspace, and apply the check-cfg lints on it.
+        //
+        // The libc build script emits check-cfg flags only when this environment variable is set,
+        // so this line allows the use of custom libcs.
+        cargo.env("LIBC_CHECK_CFG", "1");
+
         if source_type == SourceType::InTree {
             let mut lint_flags = Vec::new();
             // When extending this list, add the new lints to the RUSTFLAGS of the