Define NULL if it's not defined already.

diff --git a/include/msvcrt/stdlib.h b/include/msvcrt/stdlib.h
index 8bdf608..0ace665 100644
--- a/include/msvcrt/stdlib.h
+++ b/include/msvcrt/stdlib.h
@@ -17,6 +17,14 @@
 # endif
 #endif
 
+#ifndef NULL
+#ifdef __cplusplus
+#define NULL  0
+#else
+#define NULL  ((void*)0)
+#endif
+#endif
+
 #ifndef MSVCRT_WCHAR_T_DEFINED
 #define MSVCRT_WCHAR_T_DEFINED
 #ifndef __cplusplus