add state variable filter
change 'pc:' prefix to 'pc-' in pc-ivl.xtm
add 'already defined' checks for defining audio globals (e.g. SAMPLERATE)
add 'already defined?' check to some more xtlang global vars
fix bug so that make-chord always returns integers
fix typo in audio_dsp: peek -> peak
compiler throws a warning (not an error) when trying to re-bind a
global value
add a 'linear fade' closure
add import audio_dsp check for libsndfile