pull: If built with --disable-http2, allow enabling via http2=1

We have a `http2=[0|1]` remote config option; let's have the
`--disable-http2` build option define the default for that.  This way
it's easy to still enable http2 for testing even if
we have it disabled by default.

Closes: #1798
Approved by: jlebon

src/libostree/ostree-repo-pull.c

@@ -2948,9 +2948,14 @@ _ostree_repo_remote_new_fetcher (OstreeRepo  *self,
   if (gzip)
     fetcher_flags |= OSTREE_FETCHER_FLAGS_TRANSFER_GZIP;
 
-  { gboolean http2 = TRUE;
+  { gboolean http2_default = TRUE;
+#ifndef BUILDOPT_HTTP2
+    http2_default = FALSE;
+#endif
+    gboolean http2;
+
     if (!ostree_repo_get_remote_boolean_option (self, remote_name,
-                                                "http2", TRUE,
+                                                "http2", http2_default,
                                                 &http2, error))
       goto out;
     if (!http2)