diff --git a/sys/build.rs b/sys/build.rs
index 924ed29f..cf2ebfb4 100644
--- a/sys/build.rs
+++ b/sys/build.rs
@@ -348,12 +348,14 @@ fn check_features(
         }
         includes_code.push_str(&format!(
             r#"
+            #ifndef {var}_is_defined
             #ifndef {var}
             #define {var} 0
             #define {var}_is_defined 0
             #else
             #define {var}_is_defined 1
             #endif
+            #endif
         "#,
             var = var
         ));