How to find out a function is contained in which header file?

[丁少衡 <dshbusiness gmail com>]

Now, I'm trying to use some gtk(or gtk related) functions. I can find their description in the document on devhelp. But I could not find which header file I should include in my source code, and what CFLAGS should I specify.
For example, the function about gconf system-- gconf_client_get_default()
Please don't tell me search on the google. I know how to search that. But I think the document should contain the information about that. It was probably my fault--I do not konw how to find it. So, any help? Many thanks!

dingshaoheng