Make libx11 optional, guard x11 code paths

It's not clear whether libwnck-based functionality will ever have a
counterpart on Wayland, so it seems pointless to replace libwnck with
libxfce4windowing at this stage.

Completes: ecd1e1a645
Related: #75, #78

src/process-window.c

@@ -15,6 +15,7 @@
 #include <unistd.h>
 #endif
 
+#ifdef HAVE_LIBX11
 #include <X11/Xlib.h>
 #include <X11/Xutil.h>
 #include <X11/Xatom.h>
@@ -22,9 +23,10 @@
 #include <X11/Xmu/WinUtil.h>
 #include <X11/cursorfont.h>
 #include <X11/Xproto.h>
+#include <gdk/gdkx.h>
+#endif
 
 #include <glib-object.h>
-#include <gdk/gdkx.h>
 #include <gtk/gtk.h>
 #include <gdk/gdkkeysyms.h>
 
@@ -91,6 +93,7 @@ filter_entry_icon_pressed_cb (GtkEntry *entry,
 	}
 }
 
+#ifdef HAVE_LIBX11
 static Window
 Select_Window (Display *dpy, int screen)
 {
@@ -181,6 +184,7 @@ xwininfo_clicked_cb (GtkButton *button __unused, gpointer user_data) {
 	}
 
 }
+#endif
 
 static void
 filter_entry_keyrelease_handler(GtkEntry *entry,
@@ -304,12 +308,13 @@ xtm_process_window_init (XtmProcessWindow *window)
 										G_CALLBACK (show_settings_dialog), window);
 
 	button = GTK_WIDGET (gtk_builder_get_object (window->builder, "button-identify"));
-	if (GDK_IS_X11_DISPLAY (gdk_display_get_default ())) {
+#ifdef HAVE_LIBX11
+	if (GDK_IS_X11_DISPLAY (gdk_display_get_default ()))
 		g_signal_connect (G_OBJECT (button), "clicked",
 						  G_CALLBACK (xwininfo_clicked_cb), window);
-	} else {
+	else
+#endif
 		gtk_widget_hide (button);
-	}
 
 	window->filter_searchbar = GTK_WIDGET (gtk_builder_get_object (window->builder, "filter-searchbar"));
 	{