plug/socket: Make GtkPlug and GtkSocket X11-specific

People agreed that it doesn't make sense to make an xembed
implementation like plug/socket does not make sense on anything but X11.
And as GdkNativeWindow is going away for multibackend portability
reasons, we'd either need API fixes in the plug-socket codee or
constrain the widgets to a single backend. We chose the latter.

gtk/Makefile.am

@@ -579,7 +579,6 @@ gtk_base_c_sources = 		\
 	gtkpango.c		\
 	gtkpapersize.c		\
 	gtkpathbar.c		\
-	gtkplug.c		\
 	gtkprintcontext.c	\
 	gtkprintoperation.c	\
 	gtkprintoperationpreview.c \
@@ -615,7 +614,6 @@ gtk_base_c_sources = 		\
 	gtksizegroup.c		\
 	gtksizerequest.c	\
 	gtkshow.c		\
-	gtksocket.c		\
 	gtkspinbutton.c		\
 	gtkspinner.c		\
 	gtkstatusbar.c		\
@@ -725,7 +723,9 @@ gtk_c_sources += $(gtk_os_win32_c_sources)
 endif
 
 gtk_use_x11_c_sources = 	\
+	gtkplug.c		\
 	gtkplug-x11.c		\
+	gtksocket.c		\
 	gtksocket-x11.c		\
 	gtkxembed.c		\
 	gtktrayicon-x11.c	\

gtk/gtkplug.h

@@ -30,6 +30,10 @@
 #ifndef __GTK_PLUG_H__
 #define __GTK_PLUG_H__
 
+#include <gdk/gdk.h>
+
+#ifdef GDK_WINDOWING_X11
+
 
 #include <gtk/gtksocket.h>
 #include <gtk/gtkwindow.h>
@@ -98,4 +102,6 @@ void _gtk_plug_remove_from_socket (GtkPlug   *plug,
 
 G_END_DECLS
 
+#endif /* GDK_WINDOWING_X11 */
+
 #endif /* __GTK_PLUG_H__ */

gtk/gtksocket.h

@@ -30,6 +30,10 @@
 #ifndef __GTK_SOCKET_H__
 #define __GTK_SOCKET_H__
 
+#include <gdk/gdk.h>
+
+#ifdef GDK_WINDOWING_X11
+
 #include <gtk/gtkcontainer.h>
 
 G_BEGIN_DECLS
@@ -78,4 +82,6 @@ GdkWindow*      gtk_socket_get_plug_window (GtkSocket       *socket_);
 
 G_END_DECLS
 
+#endif /* GDK_WINDOWING_X11 */
+
 #endif /* __GTK_SOCKET_H__ */