New visibility handling in gdk

Change the visibility handling to be the same way we do it in
GLib now. We pass -fvisibility=hidden to gcc and decorate public
functions with __attribute__((visibility("default"))).

This commit just does this for GDK, GTK+ will follow later.

gdk/quartz/Makefile.am

@@ -9,6 +9,7 @@ AM_CPPFLAGS = 				\
 	-I$(top_srcdir)			\
 	-I$(top_srcdir)/gdk		\
 	-I$(top_builddir)/gdk		\
+	$(GDK_HIDDEN_VISIBILITY_CFLAGS) \
 	$(GTK_DEBUG_FLAGS) 		\
 	$(GDK_DEP_CFLAGS)		\
 	"-xobjective-c"