refactor: Add OS check to build flag to make it a bit more robust. Us…

…ing a user provided ENV variable for "extra build flags" may be a better long term solution.

pqcrypto-internals/build.rs

@@ -57,8 +57,12 @@ fn main() {
             .compile("keccak4x");
         println!("cargo:rustc-link-lib=keccak4x")
     } else if target_arch == "aarch64" && target_env != "msvc" {
+        let arch_flag = match target_os.as_str() {
+            "ios" => "-march=armv8-a+sha3",
+            _ => "-march=armv8-a",
+        };
         builder
-            .flag("-march=armv8-a+sha3")
+            .flag(arch_flag)
             .file(cfiledir.join("keccak2x").join("fips202x2.c"))
             .file(cfiledir.join("keccak2x").join("feat.S"))
             .compile("keccak2x");