This reverts part of 573e887a4c, see
https://github.com/wxWidgets/wxWidgets/pull/666 because it broke the use
of the generic dialogs in the sample under MSW (they were not used any
more, even if they were supposed to) and the explanation in the original
commit message doesn't seem correct: the WXUSINGDLL check is precisely
supposed to check if we're using a static build of wxWidgets (presumably
because of DLL export complications when not using it) and seems to work
as expected.
This allows to test generic dialogs in the sample again under MSW.
6415bd553f