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.