Allow direct use of X11 to be conditionally compiled out.

Allow direct use of X11 to be conditionally compiled out.

A major aim of introducing GTK 3 support is to permit compiling for
non-X11 platforms that GTK 3 supports, so I'm going to need to be able
to build as a pure GTK application with no use of X11 internals.
Naturally, I don't intend to stop supporting the hybrid GTK+X11 mode
in which X server-side bitmap fonts are available.

Use of X11 can be removed by compiling with -DNOT_X_WINDOWS. That's
the same compatibility flag that was already used by the unfinished OS
X port to disable the X-specific parts of uxpty.c; now it just applies
to more source files.

(There's no 'configure' option to set this flag at present. I haven't
worked out whether we'll need one yet.)