Sigh, Ruby. I just wanted to write a blog post, I didn't want to have to reinstall all of the Ruby dependencies because the installed gems no longer work with my system Ruby, and then discover that the new ones don't compile.

EDIT: It looks as if no gems that have native things can build with the latest Ruby in Homebrew for macOS because it ships a complete nonsense header that tries to include a non-existent system header, but which has the same name as the header that it includes (but included with #include not #include_next). You can comment out that line and make it work but I have no idea how that made it past code review or testing.