win64: use new headers from mingw

include/varargs.h

@@ -1,11 +1,12 @@
+/**
+ * This file has no copyright assigned and is placed in the Public Domain.
+ * This file is part of the w64 mingw-runtime package.
+ * No warranty is given; refer to the file DISCLAIMER within this package.
+ */
 #ifndef _VARARGS_H
 #define _VARARGS_H
 
-#include <stdarg.h>
-
-#define va_dcl
-#define va_alist __va_alist
-#undef va_start
-#define va_start(ap) ap = __builtin_varargs_start
+#error "TinyCC no longer implements <varargs.h>."
+#error "Revise your code to use <stdarg.h>."
 
 #endif