[pan2] Use xdg-open instead of deprecated gnome-open

[Petr Kovář <pmkovar src gnome org>]

commit 25e2b0af0dee0931fbf992ee4c381cacde64415a
Author: Petr Kovar <pknbe volny cz>
Date:   Mon Nov 27 22:50:21 2017 +0100

    Use xdg-open instead of deprecated gnome-open

 pan/gui/url.cc |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/pan/gui/url.cc b/pan/gui/url.cc
index ec5e54a..1837594 100644
--- a/pan/gui/url.cc
+++ b/pan/gui/url.cc
@@ -109,7 +109,7 @@ URL :: open (const Prefs& prefs, const char * url, Mode mode)
   const char * custom_fallback = mode==WEB ? "firefox" : "thunderbird";
   std::string cmd;
   const std::string env (prefs.get_string (mode_key, get_environment()));
-       if (env == "gnome")   cmd = "gnome-open";
+       if (env == "gnome")   cmd = "xdg-open";
   else if (env == "kde")     cmd = "kfmclient exec";
   else if (env == "mac")     cmd = "open";
   else if (env == "windows") cmd = "rundll32 url.dll,FileProtocolHandler";