How to adjust the PKG_CONFIG_PATH environment variable, while running ./configure

How to adjust the PKG_CONFIG_PATH environment variable, while running ./configure

I'm running into this error message while trying to install from a tarball
configure: error: Package requirements (gtk+-2.0 >= 2.16.0) were not met:

No package 'gtk+-2.0' found

Consider adjusting the PKG_CONFIG_PATH environment variable if you
installed software in a non-standard prefix.
Following command gives me the location of GTK
$ whereis gtk-2.0

gtk-2: /etc/gtk-2.0 /usr/lib64/gtk-2.0
My question is how to adjust PKG_CONFIG_PATH, many of the online tutorials did not make much sense to me. I'm running centos 6.4
Thanks