Test --disable-buffer-useinline in GitLab CI
#2031 (closed) would have been caught early if we were testing the
--enable-buffer-useinline compile-time switch in GitLab CI. We should
probably make one build job use it - maybe the CentOS 8 one?
One obstacle is that such a build does not currently work with
--enable-developer as the warnings issued are turned into errors.
These warnings should thus be addressed first.