Always include wx/msw/winundef.h from wx/defs.h, not just once

Fix compilation/link problems due to symbols redefinitions in user code
that could happen if it included some wx header first, then <windows.h>
(possibly indirectly, e.g. via another third party library) and then
another wx header -- in this case, the second wx header wasn't protected
from <windows.h> redefinitions resulting in all the usual problems.

Avoid this by always including winundef.h whenever any wx header is
included, not just when wx/defs.h is included for the first time.

include/wx/defs.h

@@ -10,6 +10,31 @@
 
 /* THIS IS A C FILE, DON'T USE C++ FEATURES (IN PARTICULAR COMMENTS) IN IT */
 
+/*
+    We want to avoid compilation and, even more perniciously, link errors if
+    the user code includes <windows.h> before include wxWidgets headers. These
+    error happen because <windows.h> #define's many common symbols, such as
+    Yield or GetClassInfo, which are also used in wxWidgets API. Including our
+    "cleanup" header below un-#defines them to fix this.
+
+    Moreover, notice that it is also possible for the user code to include some
+    wx header (this including wx/defs.h), then include <windows.h> and then
+    include another wx header. To avoid the problem for the second header
+    inclusion, we must include wx/msw/winundef.h from here always and not just
+    during the first inclusion, so it has to be outside of _WX_DEFS_H_ guard
+    check below.
+ */
+#ifdef __cplusplus
+    /*
+        Test for WIN32, defined by windows.h itself, not our own __WINDOWS__,
+        which is not defined yet.
+     */
+#   ifdef WIN32
+#       include "wx/msw/winundef.h"
+#   endif /* WIN32 */
+#endif /* __cplusplus */
+
+
 #ifndef _WX_DEFS_H_
 #define _WX_DEFS_H_
 
@@ -3294,16 +3319,6 @@ typedef const void* WXWidget;
 #include "wx/qt/defs.h"
 #endif
 
-/*  This is required because of clashing macros in windows.h, which may be */
-/*  included before or after wxWidgets classes, and therefore must be */
-/*  disabled here before any significant wxWidgets headers are included. */
-#ifdef __cplusplus
-#ifdef __WINDOWS__
-#include "wx/msw/winundef.h"
-#endif /* __WINDOWS__ */
-#endif /* __cplusplus */
-
-
 /*  include the feature test macros */
 #include "wx/features.h"