Remove stable_hash feature and use environment variable.

build.rs

@@ -0,0 +1,27 @@
+use std::{env, fs::File, io::Write};
+
+const WRITE_ERROR: &str = "cannot write to `config.rs`";
+
+fn main() {
+    // Tell Cargo that if the given environment variable changes, to rerun this build script.
+    println!("cargo:rerun-if-env-changed=RHAI_AHASH_SEED");
+
+    let mut f = File::create("src/config.rs").expect("cannot create `config.rs`");
+
+    f.write_fmt(format_args!(
+        "//! Configuration settings for this Rhai build
+#![allow(dead_code)]
+
+"
+    ))
+    .expect(WRITE_ERROR);
+
+    let seed = env::var("RHAI_AHASH_SEED").map_or_else(|_| "None".into(), |s| format!("Some({s})"));
+
+    f.write_fmt(format_args!(
+        "pub const AHASH_SEED: Option<[u64; 4]> = {seed};\n"
+    ))
+    .expect(WRITE_ERROR);
+
+    f.flush().expect("cannot flush `config.rs`");
+}