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.

Modules/FindwxWidgets.cmake

@@ -740,7 +740,8 @@ else()
     # UNIX: Start actual work.
     #-----------------------------------------------------------------
     # Support cross-compiling, only search in the target platform.
-    find_program(wxWidgets_CONFIG_EXECUTABLE wx-config wx-config-3.0
+    find_program(wxWidgets_CONFIG_EXECUTABLE
+      NAMES wx-config wx-config-3.0 wx-config-2.9 wx-config-2.8
       DOC "Location of wxWidgets library configuration provider binary (wx-config)."
       ONLY_CMAKE_FIND_ROOT_PATH
       )