Make Emacs build with some other XLib implementations

Make Emacs build with some other XLib implementations

* configure.ac (HAVE_XKB): Check for functions that aren't
always implemented by various XLib implementations.

* src/xfns.c (select_visual): Handle NULL values of vinfo.
(XkbRefreshKeyboardMapping):
(XkbFreeNames):
(XDisplayCells):
(XDestroySubwindows): Define replacement functions where they
aren't available.

* src/xterm.c (x_find_modifier_meanings): Handle NULL values of
various fields.