Update build.rs

Author: Berrysoft

compiler/rustc_llvm/build.rs

@@ -191,10 +191,6 @@ fn main() {
         cfg.flag(flag);
     }
 
-    if target.contains("cygwin") {
-        cfg.define("_GNU_SOURCE", None);
-    }
-
     for component in &components {
         let mut flag = String::from("LLVM_COMPONENT_");
         flag.push_str(&component.to_uppercase());