Honor RUSTFLAGS's target-cpu if given

build.rs

@@ -158,9 +158,27 @@ fn write_tables() {
     not(any(target_os = "android", target_os = "ios"))
 ))]
 fn compile_simd_c() {
+    fn guess_target_cpu() -> String {
+        // Copied and adapted from https://github.com/alexcrichton/proc-macro2/blob/4173a21dc497c67326095e438ff989cc63cd9279/build.rs#L115
+        // Licensed under Apache-2.0 + MIT (compatible because we're Apache-2.0)
+        let rustflags = env::var_os("RUSTFLAGS");
+        if let Some(rustflags) = rustflags {
+            for mut flag in rustflags.to_string_lossy().split(' ') {
+                if flag.starts_with("-C") {
+                    flag = &flag["-C".len()..];
+                }
+                if flag.starts_with("target-cpu=") {
+                    return flag["target-cpu=".len()..].to_owned()
+                }
+            }
+        }
+
+        "native".to_string()
+    }
+
     cc::Build::new()
         .opt_level(3)
-        .flag("-march=native")
+        .flag(&format!("-march={}", guess_target_cpu()))
         .flag("-std=c11")
         .file("simd_c/reedsolomon.c")
         .compile("reedsolomon");