Hello,
If I'm not mistaken, you are trying to downgrade your glib from 1.2.10 to 1.2.6 ... You might consider using up2date (or apt-get).
up2date -u glib
But if you want to compile it, rpm uses a proprietary system, so if you compile software yourself, you'll be going behind the rpm system's back, so it will still think it has one version even though you've installed another.
John