Allow `libnghttp2` to be a subproject
Subprojects can't use cc.has_function
but that's not necessary here, as nghttp2_option_set_no_rfc9113_leading_and_trailing_ws_validation
was simply introduced with version 1.50.0.
https://github.com/nghttp2/nghttp2/commit/eb06e33e383e839e39f3310a9f58ff8e51b4e224