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

docs/doxygen/overviews/envvars.h

@@ -32,5 +32,10 @@ wxWidgets programs.
          set it to @c "CURL" to force using libcurl-based implementation under
          MSW or macOS platforms where the native implementation would be chosen
          by default.}
+@itemdef{WXSUPPRESS_SIZER_FLAGS_CHECK,
+         If set, disables asserts about using invalid sizer flags in the code.
+         This can be helpful when running older programs recompiled with
+         wxWidgets 3.1 or later, as these asserts are mostly harmless and can
+         be safely ignored if the code works as expected.}
 */