Rearrange Windows checks in wx/platform.h to be more logical.

Use __WINDOWS__ symbol as the primary symbol indicating Windows instead of
using all of _WIN32, __WIN32__ and __WINDOWS__.

Also automatically define __WINDOWS__ if __WXMSW__ is defined as this port
only can be used under Windows and doing it like this fixes compilation in the
case when the platform ends up being not defined at all, closes #15342.

Finally, don't assume Windows by default but give an error if we can't detect
the platform. This shouldn't happen in practice but seems a safer thing to do
if it ever does happen.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74581 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775