Have you already installed the new glib? If your existing one is installed via rpm, it may be tricky trying to remove it and still keep the necessary dependencies intact.
Run 'rpm -qa | grep glib' in a terminal window to see a list of all installed rpms with 'glib' in their name. You can remove an rpm with 'rpm -e glib' and the like, but that'll probably complain about all the things that depend on glib.
If you can get ahold of an rpm for the glib version you want, you can use the rpm upgrade command to upgrade your existing glib. After downloading the rpm for the new version, open up a terminal window and type:
rpm -Uvh glib-new-version.rpm
or
rpmu -Uvh glib-new-version.rpm
However, it's very likely that there are a lot of dependencies that will need to be resolved before the command will actually work. It'll probably give you a list of "xyz is needed by abc" messages. Unless Redhat has officially made the RPMs available so that you can do an upgrade using one of the various graphical RPM interfaces, it could be nasty. You may be better off trying to pin down why pkg-config is seeing the old one.
Also, if you haven't tried it already, check out the program
checkinstall. It can build RPMs from source, which you may be able to use for the upgrade process described above. Personally, I'd just suggest finding everything on
rpmfind.net, though.