Don't build in-tree llvm-libunwind if system-llvm-libunwind is enable

Author: 12101111

library/unwind/build.rs

@@ -4,6 +4,10 @@ fn main() {
     println!("cargo:rerun-if-changed=build.rs");
     let target = env::var("TARGET").expect("TARGET was not set");
 
+    if cfg!(feature = "system-llvm-libunwind") {
+        return;
+    }
+
     if cfg!(feature = "llvm-libunwind")
         && ((target.contains("linux") && !target.contains("musl")) || target.contains("fuchsia"))
     {