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.

docs/changes.txt

@@ -27,7 +27,9 @@ Changes in behaviour not resulting in compilation errors
 - Using invalid flags with wxBoxSizer or wxGridSizer items now triggers asserts
   when done from the code or error messages when done in XRC. These asserts are
   best avoided by fixing the flags, but wxSizerFlags::DisableConsistencyChecks()
-  can be used to globally suppress them until this can be done.
+  can be used to globally suppress them until this can be done. Even less
+  intrusively, environment variable WXSUPPRESS_SIZER_FLAGS_CHECK can be set (to
+  any value) to achieve the same effect.
 
 - wxWS_EX_VALIDATE_RECURSIVELY is now the default behaviour, i.e. calling
   Validate() or TransferData{From,To}Window() will now also call the same