"Compiling against headers is a fairly standard autotools technique for writing portable software"

I agree entirely. But are you arguing that, in general when a program Z links to a library L to run, it is not important for the license of Z and the license of L to both permit it? A conclusion like this eviscerates all Copyleft licenses, so I am loathe to accept it.

I have harped on the close relationship between the organization of zfsonlinux's zfs.git and the linux kernel in multiple threads, because the core claim coming from Canonical is that the code in zfs.git is somehow essentially independent of the linux kernel. They do not articulate what this means or how we can determine whether it's the case in general. So far no definition I've considered seems to paint zfsonlinux in the same light that canonical have claimed.