bug#16031: references to X windows (sic)

[Ivan Shmakov]

        As of babc40c41c5c, Emacs uses “X windows” when refering to the
        X Window System (also known as X Window or simply X) in
        user-visible output (mostly documentation strings.)  I believe
        that these references should use one of the proper names
        instead.

        The files affected are as follows.

configure.ac
lisp/select.el
lisp/startup.el
lisp/term/ns-win.el
lisp/textmodes/artist.el
lisp/w32-common-fns.el
src/dispnew.c
src/frame.c
src/keyboard.c
src/w16select.c
src/w32fns.c
src/w32select.c
src/xfns.c

$ grep -irnFC1 --include=\*.{ac,c,el} -- 'X windows' 

        As an aside, when ${x_includes} and ${x_libraries} are ‘NONE’,
        configure produces:

  Where do we find X Windows header files?                NONE
  Where do we find X Windows libraries?                   NONE

        There, NOWHERE seems somehow more appropriate.

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