Do not fail to compile if autocfg unable to determine rustc version

crossbeam-utils/build.rs

@@ -1,5 +1,20 @@
+use autocfg::AutoCfg;
+
+// The rustc-cfg strings below are *not* public API. Please let us know by
+// opening a GitHub issue if your build environment requires some way to enable
+// these cfgs other than by executing our build script.
 fn main() {
-    let cfg = autocfg::new();
+    let cfg = match AutoCfg::new() {
+        Ok(cfg) => cfg,
+        Err(e) => {
+            println!(
+                "cargo:warning=crossbeam-utils: unable to determine rustc version: {}",
+                e
+            );
+            return;
+        }
+    };
+
     cfg.emit_type_cfg("core::sync::atomic::AtomicU8", "has_atomic_u8");
     cfg.emit_type_cfg("core::sync::atomic::AtomicU16", "has_atomic_u16");
     cfg.emit_type_cfg("core::sync::atomic::AtomicU32", "has_atomic_u32");