This replaces SDL threads with real cooperative threads, which are less cpu intensive and allow priority scheduling.
The backend for context switching is dependant on the host (sigaltstack/longjmp on Unix, Fibers on Windows).
configure has options to force or disallow SDL threads.
git-svn-id: svn://svn.rockbox.org/rockbox/trunk@29327 a1c6a512-1295-4272-9138-f99709370657
