Fix VC6 compilation which was somehow broken by adding wxWeakRef ctor.

For some mysterious reason, adding non-template wxWeakRef ctor breaks VC6
build with the apparently completely unrelated errors:

include\wx/toplevel.h(223) : error C2679: binary '=' : no operator defined which takes a right-hand operand of type 'class wxWindow *' (or there is no acceptable conversion)
include\wx/toplevel.h(231) : error C2679: binary '=' : no operator defined which takes a right-hand operand of type 'class wxWindow *' (or there is no acceptable conversion)
src\common\dummy.cpp(27) : error C2856: #pragma hdrstop cannot be inside an #if block

Simply don't define it for VC6 to at least allow the code not using wxWeakRef
to compile.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63955 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/weakref.h

@@ -211,12 +211,16 @@ public:
     // Default ctor
     wxWeakRef() { }
 
+    // Enabling this ctor for VC6 results in mysterious compilation failures in
+    // wx/window.h when assigning wxWindow pointers (FIXME-VC6)
+#ifndef __VISUALC6__
     // Ctor from the object of this type: this is needed as the template ctor
     // below is not used by at least g++4 when a literal NULL is used
     wxWeakRef(T *pobj)
     {
         Assign(pobj);
     }
+#endif // !__VISUALC6__
 
     // When we have the full type here, static_cast<> will always work
     // (or give a straight compiler error).