Bracket calls to timer callbacks with fl_lock_function()/fl_unlock_function() as occurs under WIN32 and X11.
This was also missing in the FLTK 1.1 carbon version.

git-svn-id: http://seriss.com/public/fltk/fltk/branches/branch-1.3@10019 ea41ed52-d2ee-0310-a9c1-e6b18d33e121