Normalize and document private header guards

HACKING.md

@@ -131,6 +131,17 @@ Header Inclusion Guards
 Guard header inclusion with `#pragma once` rather than the traditional
 `#ifndef`-`#define`-`#endif` trio.
 
+Internal headers (for consistency, whether they need to be installed or not)
+should contain the following guard to prevent users from directly including
+them:
+```c
+#if !defined(_HANDY_INSIDE) && !defined(HANDY_COMPILATION)
+#error "Only <handy.h> can be included directly."
+#endif
+```
+
+Only after these should you include headers.
+
 
 Signals
 -------

src/hdy-column.h

@@ -6,6 +6,10 @@
 
 #pragma once
 
+#if !defined(_HANDY_INSIDE) && !defined(HANDY_COMPILATION)
+#error "Only <handy.h> can be included directly."
+#endif
+
 #include <gtk/gtk.h>
 
 G_BEGIN_DECLS

src/hdy-main-private.h

@@ -5,12 +5,12 @@
  */
 #pragma once
 
-#include "hdy-main.h"
-
 #if !defined(_HANDY_INSIDE) && !defined(HANDY_COMPILATION)
 #error "Only <handy.h> can be included directly."
 #endif
 
+#include "hdy-main.h"
+
 G_BEGIN_DECLS
 
 /* Initializes the public GObject types, which is needed to ensure they are

src/hdy-main.h

@@ -5,12 +5,12 @@
  */
 #pragma once
 
-#include <glib.h>
-
 #if !defined(_HANDY_INSIDE) && !defined(HANDY_COMPILATION)
 #error "Only <handy.h> can be included directly."
 #endif
 
+#include <glib.h>
+
 G_BEGIN_DECLS
 
 gboolean hdy_init(int *argc, char ***argv);

src/hdy-title-bar.h

@@ -6,6 +6,10 @@
 
 #pragma once
 
+#if !defined(_HANDY_INSIDE) && !defined(HANDY_COMPILATION)
+#error "Only <handy.h> can be included directly."
+#endif
+
 #include <gtk/gtk.h>
 
 G_BEGIN_DECLS