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-fetcher-curl.c

@@ -777,7 +777,6 @@ initiate_next_curl_request (FetcherRequest *req,
    * there are numerous HTTP/2 fixes since the original version in
    * libcurl 7.43.0.
    */
-#ifdef BUILDOPT_HTTP2
   if (!(self->config_flags & OSTREE_FETCHER_FLAGS_DISABLE_HTTP2))
     {
 #if CURL_AT_LEAST_VERSION(7, 51, 0)
@@ -789,7 +788,7 @@ initiate_next_curl_request (FetcherRequest *req,
       curl_easy_setopt (req->easy, CURLOPT_PIPEWAIT, 1L);
 #endif
     }
-#endif
+
   curl_easy_setopt (req->easy, CURLOPT_WRITEFUNCTION, write_cb);
   if (g_getenv ("OSTREE_DEBUG_HTTP"))
     curl_easy_setopt (req->easy, CURLOPT_VERBOSE, 1L);