Try the following: # /usr/bin/gtk-config --cflags -I/usr/X11R6/include -I/usr/lib/glib/include The directory /usr/lib/glib/include does not exist in a fresh install of Red Hat 6.0.
Install the glib-devel package. It is listed as a dependency of the gtk+-devel package. If you override dependencies, you are on your own.