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diff -ur mingw-w64-v3.3.0.orig/mingw-w64-tools/widl/include/windef.h mingw-w64-v3.3.0/mingw-w64-tools/widl/include/windef.h
--- mingw-w64-v3.3.0.orig/mingw-w64-tools/widl/include/windef.h	2014-10-29 20:53:30.000000000 +1100
+++ mingw-w64-v3.3.0/mingw-w64-tools/widl/include/windef.h	2014-10-29 20:54:09.000000000 +1100
@@ -85,7 +85,7 @@
 #endif /* __cdecl */
 
 #ifndef __ms_va_list
-# if defined(__x86_64__) && defined (__GNUC__)
+# if defined(__x86_64__) && defined (__GNUC__) && defined(__builtin_ms_va_list)
 #  define __ms_va_list __builtin_ms_va_list
 #  define __ms_va_start(list,arg) __builtin_ms_va_start(list,arg)
 #  define __ms_va_end(list) __builtin_ms_va_end(list)