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)"
Merge pull request #1788 from embray/fix-system-cfitsio

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