Fix build failure at -DNOT_X_WINDOWS.

I had been indecisive about whether the definitions and calls of
store_cutbuffer and retrieve_cutbuffer should be compiled out
completely in GTK-without-X mode, or whether the definitions should be
left in as stubs and the calls still present. retrieve_cutbuffer ended
up with a definition but no call in that mode.

It was only an unused-function warning, but -Werror promoted it to an
error. Fixed by making up my mind: now the functions are completely
absent, and so are the calls to them.

CHECKLST.txt

@@ -115,6 +115,7 @@ Making a release candidate build
        + on at least a reasonably current stable Linux distro, and
          also try Debian sid
        + test-build with all of GTK 1, 2 and 3
+       + test-build with -DNOT_X_WINDOWS
     * feed the release-candidate source to Coverity and make sure it
       didn't turn up any last-minute problems
     * make sure we have a clean run of sctest

unix/gtkwin.c

@@ -2880,16 +2880,16 @@ static void gtkwin_clip_request_paste(TermWin *tw, int clipboard)
  * GtkTargetList.) But that work can wait until there's a need for it!
  */
 
+#ifndef NOT_X_WINDOWS
+
 /* Store the data in a cut-buffer. */
 static void store_cutbuffer(GtkFrontend *inst, char *ptr, int len)
 {
-#ifndef NOT_X_WINDOWS
     if (inst->disp) {
         /* ICCCM says we must rotate the buffers before storing to buffer 0. */
         XRotateBuffers(inst->disp, 1);
         XStoreBytes(inst->disp, ptr, len);
     }
-#endif
 }
 
 /* Retrieve data from a cut-buffer.
@@ -2897,7 +2897,6 @@ static void store_cutbuffer(GtkFrontend *inst, char *ptr, int len)
  */
 static char *retrieve_cutbuffer(GtkFrontend *inst, int *nbytes)
 {
-#ifndef NOT_X_WINDOWS
     char *ptr;
     if (!inst->disp) {
         *nbytes = 0;
@@ -2909,12 +2908,12 @@ static char *retrieve_cutbuffer(GtkFrontend *inst, int *nbytes)
         ptr = 0;
     }
     return ptr;
-#else
     *nbytes = 0;
     return NULL;
-#endif
 }
 
+#endif /* NOT_X_WINDOWS */
+
 static void gtkwin_clip_write(
     TermWin *tw, int clipboard, wchar_t *data, int *attr,
     truecolour *truecolour, int len, bool must_deselect)
@@ -2995,9 +2994,11 @@ static void gtkwin_clip_write(
             sresize(state->pasteout_data, state->pasteout_data_len, char);
     }
 
+#ifndef NOT_X_WINDOWS
     /* The legacy X cut buffers go with PRIMARY, not any other clipboard */
     if (state->atom == GDK_SELECTION_PRIMARY)
         store_cutbuffer(inst, state->pasteout_data, state->pasteout_data_len);
+#endif
 
     if (gtk_selection_owner_set(inst->area, state->atom,
                                 inst->input_event_time)) {