Reconcile HAVE_VARIADIC_MACROS and wxHAS_VARIADIC_MACROS definitions.

The main user-visible effect of this change is that giving configure
--disable-vararg-macros argument now really disables the use of variadic
macros whereas it didn't disable them in wx/cpp.h before.

It is, of course, also less confusing to not have two very similar but
different symbols.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74489 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775