Switching to build in -werror in 4016c839a8
was not working properly. Adjust the CI builds to use --werror instead.
Also while doing this, enable -werror in clang builds.
Automake and autoconf are impossible to fully understand and Meson now provides
everything we need for a much smaller, faster, and more understandable build.
See http://mesonbuild.com/ for more information.