Define HAVE_VARIADIC_MACROS as 1, not nothing, in wx/cpp.h.

This avoids g++ warning about HAVE_VARIADIC_MACROS redefinition in setup.h if
wx/cpp.h happens to be included before it.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75519 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Move UNICODE section of wx/platform.h before wx/compiler.h inclusion.

This fixes the build with some versions of MinGW which depend on UNICODE being
defined correctly in _mingw.h which is included from wx/compiler.h, so that
before this change the build was broken as inconsistent definitions of UNICODE
were used there and in windows.h included later.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75520 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775