Enable use of thread safety attributes in project

Change-Id: I6449171a37a084a75de1ba486ae2b0592e3c9242
diff --git a/BUILD.gn b/BUILD.gn
index c1cb3b1..c5aa2df 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -25,5 +25,8 @@
     "src/timers.cc",
   ]
   public_configs = [ ":benchmark_config" ]
-  defines = [ "HAVE_POSIX_REGEX" ]
+  defines = [
+    "HAVE_POSIX_REGEX",
+    "HAVE_THREAD_SAFETY_ATTRIBUTES",
+  ]
 }