Thread-safety changes to signal marshalling

[Torsten Schoenfeld <kaffeetisch gmx de>]

I just committed¹ a fix to Glib that is supposed to make Glib gracefully 
handle signal invocation from foreign threads.  The long story is at 
<https://bugzilla.gnome.org/show_bug.cgi?id=620099>, the short story is 
in the NEWS file:

When a signal handler is invoked from a foreign thread without 
associated Perl interpreter, hand the marshalling over to the main loop 
which in turn later wakes up the main thread and lets it handle the 
request.  Since this approach is experimental, there is debug print for 
now whenever it is used:

  *** GPerl asked to invoke callback from a foreign thread;
      handing it over to the main loop

Please test this change extensively.  The programs that are most likely 
affected are those using GStreamer, but maybe also those using gio.

¹ 
<http://git.gnome.org/browse/perl-Glib/commit/?id=6f1bc1a56e4c7c87d44caca1001cd50357f7a603>