Let CFLAGS from environment take precedence over internal defined values

darcs-hash:20090112164837-b22ec-2ec38239e2b391b14adc11d139c0db0e2e8f71d3
Added Cython version check to setup script

darcs-hash:20090112163757-58103-75f0848c72e7b9938e228991bf54be4167efeb52