FindwxWidgets: Fix finding unversioned VS-built directory prefixes

The change in commit v3.7.0-rc1~217^2 (FindwxWidgets: Add VS-versioned
library directory prefixes, 2016-08-24) works only for official
wxWidgets builds that add the VS-versioned directory prefixes.  Local
wxWidgets builds still have an unversioned prefix.  Search them.

Closes: #16366