Always define SIZEOF_WCHAR_T if it's not defined under Windows.

Always define SIZEOF_WCHAR_T if it's not defined under Windows.

The assumption that SIZEOF_EVERYTHING_ELSE is defined when SIZEOF_INT is is
wrong. While wxWidgets configure does define all SIZEOF_XXX at once,
SIZEOF_INT could also be defined in some third party headers (e.g. Python.h)
and we still need to define the other ones.

In particular, doing this fixes the problem with SIZEOF_WCHAR_T not being
defined during wxPython build.

Closes #12013.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64373 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775