Specify consistent c++ standard between cxx and cxx-test-suite
diff --git a/tests/ffi/build.rs b/tests/ffi/build.rs
index f6fa59e..8042129 100644
--- a/tests/ffi/build.rs
+++ b/tests/ffi/build.rs
@@ -6,6 +6,6 @@
     let sources = vec!["lib.rs", "module.rs"];
     cxx_build::bridges(sources)
         .file("tests.cc")
-        .flag_if_supported("-std=c++11")
+        .flag_if_supported(cxxbridge_flags::STD)
         .compile("cxx-test-suite");
 }