ci: Make build stage fail on any compiler warning

Including on glib deprecation warnings, which are probably the only ones we care about.

As said in comment, this should a priori be achieved using -Werror, but it interacts badly with autotools (at least). For example, it breaks macros such as AC_CHECK_LIB, or compiler checks, in short, anything that runs a bit of code likely to generate warnings. And it causes other link errors that I haven't been able to solve (for example, on a project like xfce4-docklike-plugin).

This doesn't seem to be well documented, see e.g. https://squidproxy.wordpress.com/2024/03/05/the-unintended-consequences-of-using-cflags-werror-with-configure-and-gcc/