Experimental checkin: re-enable support for the static WM_MOUSEWHEEL

Experimental checkin: re-enable support for the static WM_MOUSEWHEEL
even on systems where the version number check thinks we should be
expecting a dynamically registered message number. I have a
suspicion the version check isn't accurate in all circumstances; and
in any case, in a situation like this where Windows might send you
either of two things, it's better to be ready to deal with both than
to try and second-guess which one you'll get.

git-svn-id: svn://svn.tartarus.org/sgt/putty@1549 cda61777-01e9-0310-a592-d414129be87e