vquic: add support for GnuTLS backend of ngtcp2

[ueno]

This can be built with the same steps as the ngtcp2 + OpenSSL procedure mentioned in docs/HTTP3.md, except:

• gnutls (from the tmp-quic branch) needs to be installed instead

     % git clone https://gitlab.com/gnutls/gnutls.git
     % cd gnutls
     % git checkout tmp-quic
     % ./bootstrap
     % ./configure --prefix=<somewhere1> --disable-doc
     % make -j$(nproc)
     % make install

• ngtcp2 (with the gnutls crypto backend) needs to be installed afterwards

     % cd ..
     % git clone https://github.com/ueno/ngtcp2.git
     % cd ngtcp2
     % git checkout wip/dueno/crypto-gnutls
     % autoreconf -i
     % ./configure --prefix=<somewhere2> --enable-lib-only
     % make -j$(nproc)
     % make install

• curl must be configured with --with-gnutls instead of --with-ssl

     % cd ..
     % git clone https://github.com/curl/curl
     % cd curl
     % ./buildconf
     % LDFLAGS="-Wl,-rpath,<somewhere1>/lib" ./configure --without-ssl --with-gnutls=<somewhere1> --with-nghttp3=<somewhere2> --with-ngtcp2=<somewhere3> --enable-alt-svc
     % make

Currently, the TLS backend used by vquic/ngtcp2.c is selected at compile time. Therefore OpenSSL support needs to be explicitly disabled.

[bagder]

Lovely! Can you also update docs/HTTP3.md with these how-to details?

Do you have any ideas or guesses on when we can hope for this work to be merged into the master branches of GnuTLS and ngtcp2?

It think it also makes sense to create a CI job that verifies the build, in the same spirit as the existing ngtcp2-openssl build is done. We can of course do that as a second follow-up PR.

[tatsuhiro-t]

I just merged GnuTLS support into ngtcp2.

[ueno]

Thank you; updated the patches with the suggested changes.

[bagder]

Maybe you could/should make a separate PR for db45199 first to fix #5166 independently? Or does it make more sense to fix it within this PR?

[ueno]

Sure, filed a separate PR as #5168.

[ueno]

The build in Travis CI seems to have succeeded (finally): https://travis-ci.org/github/curl/curl/jobs/669087295

[bagder]

Thank you!