Improve XInput2 version checking

Improve XInput2 version checking

* configure.ac: Check for various important structures from all
versions of libXi.

* src/xfns.c (setup_xi_event_mask):
* src/xwidget.c (x_draw_xwidget_glyph_string):
* src/xterm.c (x_init_master_valuators, handle_one_xevent)
(x_term_init): Replace XI version checks based on protocol
headers with new constants.

* src/xterm.h (HAVE_XINPUT2_1, HAVE_XINPUT2_2, HAVE_XINPUT2_3)
(HAVE_XINPUT2_4): New definitions.