Also, see the following forum tree:
I have been running Gentoo for 6 years on my current laptop. During all these years I have not been always paying attention on the CONFIG_ warning messages.
The log file /var/log/portage/elog/summary.log seems to have it records, but it might be no longer relevant to my current set of installed packages.
So I'm wondering, it should be a good idea to keep thes records in portage database somewhere (something similar to @module-rebuild perhaps?) and update it during installing/uninstalling packages.
Then, user should be able to run something like "emerge @kernel_config_check" command which would display a missing kernel settings.
*** This bug has been marked as a duplicate of bug 551430 ***
*** This bug has been marked as a duplicate of bug 532674 ***