Just because two enums have the same values, doesn't mean they
should be merged.

But in this case an API of

gtk_range_set_*_stepper_sensitivity (GtkRange      *range,
                                     GtkPolicyType  sensitivity_type);

would make sense because

typedef enum
{
  GTK_POLICY_ALWAYS,
  GTK_POLICY_AUTOMATIC,
  GTK_POLICY_NEVER
} GtkPolicyType;

completely makes sense as "sensitivity policy".