This fixes problems with parallel builds in Visual Studio 2012 or 2013 IDE as
multiple projects may try to copy setup.h file at once, resulting in errors.
Use _custom_build project as a common dependency to ensure that the copy step
is done only once.
Closes#16177.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76419 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
