Update build.rs

Author: Lokathor

build.rs

@@ -9,7 +9,7 @@ fn main() {
     println!("cargo:compiler-rt={}", cwd.join("compiler-rt").display());
 
     // Activate libm's unstable features to make full use of Nightly.
-    println!("rustc-cfg=libm/unstable");
+    println!("cargo:rustc-cfg=feature=\"unstable\"");
 
     // Emscripten's runtime includes all the builtins
     if target.contains("emscripten") {