build: Add --disable-static-deltas

Since they're unstable, we want to allow organizations shipping ostree
now to be future proof against any changes.

src/libostree/ostree-repo-pull.c

@@ -1965,12 +1965,14 @@ ostree_repo_pull_with_options (OstreeRepo             *self,
                                     &from_revision, error))
         goto out;
 
+#ifdef BUILDOPT_STATIC_DELTAS
       if (from_revision == NULL || g_strcmp0 (from_revision, to_revision) != 0)
         {
           if (!request_static_delta_superblock_sync (pull_data, from_revision, to_revision,
                                                      &delta_superblock, cancellable, error))
             goto out;
         }
+#endif
           
       if (!delta_superblock)
         {