ugh. Nesting the (per class) plugin sentries can require them to

ugh.  Nesting the (per class) plugin sentries can require them to
have user export syntax under win32 (unlike the old method which
had it's own uglyness though).  Consider this a quickfix for win32,
will look at better solutions after some sleep.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@13132 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775