For what it's worth, I did raise this as a potential issue with the apache2 license migration for Khronos projects, and explicitly asked about MIT dual-licensing at least for the GL headers. (The morbidly curious with a Khronos membership may find the relevant discussion in the arb-nextgen archives from May 2016.) I am not a lawyer, I am extremely not your lawyer, and I am especially extremely not OpenBSD's lawyer, but my considered opinion is OpenBSD is just straight up misguided about this and they do themselves a disservice here. Whatever, not my circus, not my monkeys.
I don't imagine Mesa is ever going to stop using the Khronos-generated headers as we have very good reasons to want not to diverge from them, so I would encourage thinking of this as unavoidable, like it is for llvm, which you're already building the OS with, so. - ajax
