Harden dependency and supply-chain policy (#100)

crates/libremetaverse-opus/build.rs

@@ -0,0 +1,16 @@
+use std::env;
+
+fn main() {
+    println!("cargo:rerun-if-changed=build.rs");
+    let target = env::var("TARGET").expect("Cargo provides TARGET");
+    if target.contains("msvc") {
+        vcpkg::Config::new()
+            .find_package("opus")
+            .expect("WebRTC voice requires libopus 1.3 or newer from vcpkg");
+    } else {
+        pkg_config::Config::new()
+            .atleast_version("1.3")
+            .probe("opus")
+            .expect("WebRTC voice requires pkg-config and libopus 1.3 or newer");
+    }
+}