CCfits 2.4: new formula

This formula is for [`CCfits` 2.4](http://heasarc.gsfc.nasa.gov/fitsio/CCfits/), patched for OS X Mavericks as described in the [official documentation](http://heasarc.gsfc.nasa.gov/fitsio/CCfits/html/).
So it should only be applied if building with clang?!