Disable sizer flag checks if WXSUPPRESS_SIZER_FLAGS_CHECK is set

This provides a less intrusive, and also usable by the end users rather
than only by the developers, way of doing the same thing as the just
added wxSizerFlags::DisableConsistencyChecks() does.

interface/wx/sizer.h

@@ -1499,6 +1499,10 @@ public:
         using this function, especially permanently, rather than a temporary
         workaround, is @e not recommended.
 
+        Notice that the same effect as calling this function can be achieved by
+        setting the environment variable @c WXSUPPRESS_SIZER_FLAGS_CHECK to any
+        value.
+
         @since 3.1.6
      */
     static void DisableConsistencyChecks();