You are (Gentoo) linux-user.
You are building packages by yourself.
How many times you have got message “X_Y_Z not enabled in kernel config”, like this (trying to emerge iotop):
* Checking for suitable kernel configuration options... * CONFIG_TASK_IO_ACCOUNTING: is not set when it should be.
And sometimes it is pretty hard to find out, exactly where in “make menuconfig” referred config-option lays. Google can give some times answer as first item, sometimes it is not even on 10th page.
There is solution for that : http://kernel.xc.net/
Just choose your kernel version (latest selected by default), and
arch at “Complete config for” and press “Go”
For example you get page like this. All config-options in one page (Help also available) and there you can search (in browser Ctrl+F) option you need and backtrace by indentations how to find it out from menuconfig.
Other, also very useful link is http://lxr.linux.no/ – Linux kernel code cross referenced (if you need to find out where is some type defined, just press link)