Revert wxDO_LOG_IF_ENABLED() change for MSVC6 in r74735.

The old solution worked fine for that compiler and the new one fails when
wxLogXXX() is used inside a switch statement to the wrong rules used by VC6
for the scope of the variables defined inside the for loop.

Simply revert back to using the old version for it, this will be easy to
re-revert after 3.0 by simply removing the check for it.

See #11829.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74762 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Export recently added wxRichTextXMLHelper to fix link errors.

This class is referenced from outside the library, see e.g. this build log:

http://buildbot.tt-solutions.com/wx/builders/Linux x86 wxGTK trunk/builds/2961/steps/compile utils/logs/stdio

and so must be exported, otherwise the build fails under Linux (and under
Windows when using DLL).

Perhaps it would be better to make it completely private to the library if it
is not meant to be public, but this would require more changes.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74763 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775