ci: Make build stage fail on any compiler warning (xdt-features version)

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

This assumes that XDT_FEATURE_DEBUG is used on all projects where build_project.yml is included, which is currently the case.

This only applies to the build stage, as other distcheck-specific warnings may appear that we don't necessarily want to turn into errors, especially on vala projects. Also, this leaves open the possibility of applying another debug level to distcheck, and thus compiling other code paths.