Buggy interaction with flycheck-display-errors-function customization

[bcc32]

If a user customizes flycheck-display-errors-function to set it to flycheck-pos-tip-error-messages, or if they simply "save" the flycheck group customizations while flycheck-pos-tip-mode is enabled, flycheck-pos-tip will malfunction on the next Emacs session.

In particular, flycheck-pos-tip-mode will not add the correct hooks to dismiss the error tooltip because flycheck-display-errors-function is already set to the expected value.

No matter whether flycheck-pos-tip-mode is invoked on the next Emacs session, the errors are displayed by tooltip but the tooltip does not get dismissed automatically as expected.