FindwxWidgets: set wxWidgets_FIND_STYLE to unix on MINGW

The MinGW variant has wx-config which make it easier to find.

Modules/FindwxWidgets.cmake

@@ -218,7 +218,7 @@ endif()
 #=====================================================================
 # Determine whether unix or win32 paths should be used
 #=====================================================================
-if(WIN32 AND NOT CYGWIN AND NOT MSYS AND NOT CMAKE_CROSSCOMPILING)
+if(WIN32 AND NOT CYGWIN AND NOT MSYS AND NOT MINGW AND NOT CMAKE_CROSSCOMPILING)
   set(wxWidgets_FIND_STYLE "win32")
 else()
   set(wxWidgets_FIND_STYLE "unix")