diff --git a/simulator/Makefile b/simulator/Makefile
index c23d5f6fd2..c001074f48 100644
--- a/simulator/Makefile
+++ b/simulator/Makefile
@@ -19,7 +19,7 @@ endif
 
 # C specific options here (added to USE_OPT).
 ifeq ($(USE_COPT),)
-  USE_COPT = 
+  USE_COPT = -std=gnu99
 endif
 
 # C++ specific options here (added to USE_OPT).