build: Allow ostree to use g_autoptr on its types internally

As an example, I g_autoptred one OstreeRepo in the "ostree init"
command.

Closes: #295
Approved by: cgwalters

src/libostree/ostree-autocleanups.h

@@ -30,7 +30,10 @@ G_BEGIN_DECLS
 #define OSTREE_WITH_AUTOCLEANUPS 0
 #endif
 
-#if OSTREE_WITH_AUTOCLEANUPS && GLIB_CHECK_VERSION(2, 44, 0)
+/* ostree can use g_autoptr backports from libglnx when glib is too
+ * old, but still avoid exposing them to users that also have an old
+ * glib */
+#if defined(OSTREE_COMPILATION) || (OSTREE_WITH_AUTOCLEANUPS && GLIB_CHECK_VERSION(2, 44, 0))
 
 /*
  * The following types have no specific clear/free/unref functions, so

src/ostree/ot-builtin-init.c

@@ -37,7 +37,7 @@ gboolean
 ostree_builtin_init (int argc, char **argv, GCancellable *cancellable, GError **error)
 {
   GOptionContext *context = NULL;
-  glnx_unref_object OstreeRepo *repo = NULL;
+  g_autoptr(OstreeRepo) repo = NULL;
   gboolean ret = FALSE;
   OstreeRepoMode mode;