patch 9.2.0504: configure: requires X11 libraries for GTK4 build

patch 9.2.0504: configure: requires X11 libraries for GTK4 build

Problem:  configure: requires X11 libraries for GTK4 build
          (after v9.2.0501)
Solution: Allow to build GTK4 even when no X11 libraries are present
          (Yasuhiro Matsumoto)

GTK4 does not use X11 APIs directly; the X11 backend is loaded by
GTK4 at runtime. Skip the X11 dependency enforcement when the user
explicitly passes --enable-gui=gtk4 so the build can succeed on
systems without X11 development headers.

closes: #20265

Co-authored-by: Claude <noreply@anthropic.com>
Signed-off-by: Yasuhiro Matsumoto <mattn.jp@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>