The extra HTTP header fields are compiled from
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers,
https://en.wikipedia.org/wiki/List_of_HTTP_header_fields, and
https://www.owasp.org/index.php/List_of_useful_HTTP_headers.

Since v0.6.2-7-g1d138acc ("Unify DATA and other frames in
nghttp2_outbound_item and save malloc()"), the macros are unused and the
builds fails on -Werror=unused-macros.