Set the program name for --help-gtk.

gnucash/gnucash.cpp

@@ -319,6 +319,7 @@ Gnucash::Gnucash::configure_program_options (void)
     // for gtk's options. The options themselves are already parsed out by
     // gtk_init_check by the time this function is called though. So it really only
     // serves to be able to display a help message.
+    g_set_prgname ("gnucash");
     auto context = g_option_context_new (m_tagline.c_str());
     auto gtk_options = gtk_get_option_group(FALSE);
     g_option_context_add_group (context, gtk_options);