[CM] RTS improvements for SBCL/Linux

[Rick Taube]

thanks to david psenicka cm's real time scheduler now runs more  
reliably in SBCL/Linux . the main change is that the rts thread is  
now created by sbcl via a callback from c. rts now coexists amicably  
with (other) sbcl threads and multiple sbcl threads can now sprout to  
rts at the same time. its also now possible to use rr and fifo  
scheduling policy without segfaulting. david also twiddled the config/ 
make process for linux so it should build ootb more reliably.

rts timing in sbcl is pretty good -- not quite as good as on openmcl/ 
darwin but certainly very usable! here is a pdf of a 12000 note  
stress test running a musical process every 50 ms, just a few +-15ms  
spikes.

	http://pinhead.music.uiuc.edu/~hkt/rtstest/sbcl-rr.pdf