This causes a build failure under Mac OS because GtkCssNode is defined
in both gtk/gtkcssnodeprivate.h and gtk/gtkcsstypesprivate.h.
https://bugzilla.gnome.org/show_bug.cgi?id=747560
#define GTK_IS_CSS_NODE_CLASS(obj) (G_TYPE_CHECK_CLASS_TYPE (obj, GTK_TYPE_CSS_NODE))
#define GTK_CSS_NODE_GET_CLASS(obj) (G_TYPE_INSTANCE_GET_CLASS ((obj), GTK_TYPE_CSS_NODE, GtkCssNodeClass))
-typedef struct _GtkCssNode GtkCssNode;
typedef struct _GtkCssNodeClass GtkCssNodeClass;
struct _GtkCssNode