Only build the threadpool if customized

src/lib.rs

@@ -897,13 +897,13 @@ impl Build {
     fn compile_objects(&self, objs: &[Object]) -> Result<(), Error> {
         use self::rayon::prelude::*;
 
-        let mut builder = rayon::ThreadPoolBuilder::new();
         if let Ok(amt) = env::var("NUM_JOBS") {
             if let Ok(amt) = amt.parse() {
-                builder = builder.num_threads(amt);
+                let _ = rayon::ThreadPoolBuilder::new()
+                    .num_threads(amt)
+                    .build_global();
             }
         }
-        drop(builder.build_global());
 
         let results: Mutex<Vec<Result<(), Error>>> = Mutex::new(Vec::new());