Improve compatibility with MacOS X.

Author: xavierleroy

runtime/include/stdarg.h

@@ -45,7 +45,10 @@ typedef __builtin_va_list __gnuc_va_list;
 
 #ifdef _STDARG_H
 
+#ifndef _VA_LIST_T
+#define _VA_LIST_T
 typedef __builtin_va_list va_list;
+#endif
 
 #define va_start(v,l) __builtin_va_start(v,l)
 #define va_end(v) __builtin_va_end(v)