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

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