diff --git a/include/windef.h b/include/windef.h
index db2f0af2d3e..4b9800bc932 100644
--- a/include/windef.h
+++ b/include/windef.h
@@ -102,6 +102,9 @@ extern "C" {
 /* Calling conventions definitions */
 
 #ifdef __i386__
+# ifndef _X86_
+#  define _X86_
+# endif
 # if defined(__GNUC__) && ((__GNUC__ > 2) || ((__GNUC__ == 2) && (__GNUC_MINOR__ >= 7)))
 #  define __stdcall __attribute__((__stdcall__))
 #  define __cdecl   __attribute__((__cdecl__))