Oh, did you build it from source? Sorry, I just assumed you're using rpm's like the majority.
If you have locally built GTK packages and you didn't specify a prefix when you configured them you will most likely find them in /usr/local/lib. So try running the configure script as
PKG_CONFIG_PATH=$PKG_CONFIG_PATH:/usr/local/lib:/usr/local/lib/pkgconfig ./configure
Håkan