It turns out that I can't build on macOS because I can't find webkitgtk there. Homebrew used to provide it but deleted it because of build problems, apparently.
Configuring webkitgtk3-0.14.2.1...
setup: The pkg-config package 'webkitgtk-3.0' version >=1.8 is required but it
could not be found.
It turns out that I can't build on macOS because I can't find
webkitgtkthere. Homebrew used to provide it but deleted it because of build problems, apparently.