FindwxWidgets: Fix find_program call for versioned names
In commit v3.3.0-rc1~132^2 (FindwxWidgets: Search for wx-config-3.0 in addition to wx-config, 2015-04-29) we added a second (versioned) name to the find_program call. Specifying multiple names requires use of the NAMES option. Add it now. While at it, also add versioned names for 2.9 and 2.8.