Since many distributions ship libjxl 0.7.0 still, we'd still prefer to
compile against that, but don't want to lose the features that require
libjxl 0.8.0 or greater. For this reason I've added preprocessor #ifdef
guards around the features that aren't necessarily in libjxl 0.7.0.
Signed-off-by: Leo Izen <leo.izen@gmail.com>