Stop using GTK3-deprecated gdk_get_display().

The new way is gdk_display_get_name(gdk_display_get_default()), which
returns a const char * rather than a char *, so I've also had to
fiddle with the prototype and call sites of get_x_display().

(Also included gtkcompat.h into uxputty.c, since that wanted to call
gdk_get_display() but didn't previously include it.)

unix/gtkcompat.h

@@ -177,4 +177,7 @@
     gtk_box_new(GTK_ORIENTATION_VERTICAL, spacing)
 #define gtk_vscrollbar_new(adjust) \
     gtk_scrollbar_new(GTK_ORIENTATION_VERTICAL, adjust)
+
+#define gdk_get_display() gdk_display_get_name(gdk_display_get_default())
+
 #endif

unix/gtkwin.c

@@ -2998,7 +2998,7 @@ void cmdline_error(const char *p, ...)
     exit(1);
 }
 
-char *get_x_display(void *frontend)
+const char *get_x_display(void *frontend)
 {
     return gdk_get_display();
 }

unix/unix.h

@@ -74,7 +74,7 @@ unsigned long getticks(void);	       /* based on gettimeofday(2) */
 #define FLAG_STDERR_TTY 0x1000
 
 /* Things pty.c needs from pterm.c */
-char *get_x_display(void *frontend);
+const char *get_x_display(void *frontend);
 int font_dimension(void *frontend, int which);/* 0 for width, 1 for height */
 long get_windowid(void *frontend);
 

unix/uxpty.c

@@ -759,7 +759,7 @@ static const char *pty_init(void *frontend, void **backend_handle, Conf *conf,
             close(pty_utmp_helper_pipe);   /* just let the child process die */
             pty_utmp_helper_pipe = -1;
         } else {
-            char *location = get_x_display(pty->frontend);
+            const char *location = get_x_display(pty->frontend);
             int len = strlen(location)+1, pos = 0;   /* +1 to include NUL */
             while (pos < len) {
                 int ret = write(pty_utmp_helper_pipe, location+pos, len - pos);

unix/uxputty.c

@@ -7,11 +7,14 @@
 #include <stdlib.h>
 #include <assert.h>
 #include <unistd.h>
+#include <gtk/gtk.h>
 #include <gdk/gdk.h>
 
 #include "putty.h"
 #include "storage.h"
 
+#include "gtkcompat.h"
+
 /*
  * Stubs to avoid uxpty.c needing to be linked in.
  */