Let me send a patch again, to change names of two variables and
how two variables are defined, as below.

[from mwheel-tilt-scroll-p]  --> [to mouse-wheel-tilt-scroll]
[from mwheel-flip-direction] --> [to mouse-wheel-flip-direction]
[from (defcustom mwheel-scroll-left-function 'scroll-left)]
   --> [to (defvar mwheel-scroll-left-function 'scroll-left)]
[from (defcustom mwheel-scroll-right-function 'scroll-right)]
   --> [to (defvar mwheel-scroll-right-function 'scroll-right)]


* Commit log

   Change name and definition of variables to be consistent

   Variables mwheel-tilt-scroll-p and mwheel-flip-direction were
   renamed to mouse-wheel-tilt-scroll and mouse-wheel-flip-direction,
   respectively.  Variables mwheel-scroll-left-function and
   mwheel-scroll-right-function that were defined by defcustom are
   defined by defvar.  Suggested by Glenn Morris <rgm@gnu.org> in
   emacs-devel@gnu.org.

   * etc/NEWS:
   * doc/emacs/frames.texi (Mouse Commands):
   * lisp/mwheel.el (mouse-wheel-tilt-scroll):
   (mouse-wheel-flip-direction): Variables are renamed.
   * lisp/mwheel.el (mwheel-scroll-left-function):
   (mwheel-scroll-right-function): Defined as defvar.