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/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.}
 */