Fix bug when building with --use-system-cfitsio and CFITSIO < 3.35

This is required in order to build the io.fits.compression module with --use-system-cfitsio on systems where CFITSIO < 3.35 (the NO_DITHER macro is not defined in older versions, and the value used where NO_DITHER *is* used in 3.35 is 0, not -1 which is what NO_DITHER is defined in 3.35)"

Thanks @pllim for testing this.