diff --git a/posix.cc b/posix.cc
index d8f9bb62..023cd9e1 100644
--- a/posix.cc
+++ b/posix.cc
@@ -26,7 +26,9 @@
  */
 
 // Disable bogus MSVC warnings.
-#define _CRT_SECURE_NO_WARNINGS
+#ifndef _CRT_SECURE_NO_WARNINGS
+# define _CRT_SECURE_NO_WARNINGS
+#endif
 
 #include "posix.h"