Issue 666 gr audio osx fixes

A bunch of small changes, with the primary ones that should fix issue #666 regarding the gr-audio OSX sink not working -- turns out I can't throw correctly in the audio render callback from the OS.  I'm not sure if this was really the issue, but with these changes all now seems to work in the OSX audio sink.

Also, a few fixes to the gr::thread class since OSX does provide a little functionality in that regard ;)

You can cherry pick as you wish.  I tried to base off of maint ... really!  I see that GitHub does not think so ... ah well ... at least I tried ... - MLD