The command /usr/bin/glib-config from GLIB 1.2 is still present on Red Hat 6.1 and 6.2 systems (GLIB 2.x) systems. glib-config --version returns 1.2.6 for GLIB - hence misleading. Source for glib-config not found in GLIB 2.x source, but found in source for GLIB 1.2. Hence I am guessing that this command is a remnant of upgrading previous versions (rather than creating distributions from a new install).
You are most likely confusing GLib and glibc. GLib is a library of convenience routines used by GTK+, GNOME, and many other packages. glibc is the GNU C library. The current version of GLib is 1.2 and contains glib-config.