Ubuntu patch breaks libproxy

We got reports upstream of libproxy: libproxy is not working on Ubuntu.
A user, even when having a Gnome configuration set, get's a warning wpad disabled and the lib falls back to direct, bypassing his proxy.

I would say this part of your 50_px-wpad-fallback-env-var.patch is wrong:
@@ -591,7 +608,12 @@ px_proxy_factory_get_proxies (pxProxyFac
  }

  // If the config plugin returned an invalid config type or malformed URL, fall back to 'wpad://'
- if (!(!strncmp(config->url, "http://", 7) ||
+ if (!do_wpad_fallback)
+ {
+ fprintf(stderr, "*** Config plugin returned invalid URL type and WPAD fallback disabled! Falling back to direct...\n");
+ goto do_return;
+ }
+ else if (!(!strncmp(config->url, "http://", 7) ||
     !strncmp(config->url, "socks://", 8) ||
     !strncmp(config->url, "pac+", 4) ||
     !strcmp (config->url, "wpad://") ||

The only check here seems to be if do_wpad_fallback is enabled, if not give an error and fall back to direct://, without checkling if the config otherwise would have been valid.

I suggest this to && linked with the rest of the original IF.