I use app-portage/elogviewer. It has a GUI in which you can sort, filter and flag the messages.
There is also a curses based tool called app-portage/elogv in case you want to stay on the command line.
Note that in both cases you need to enable and select elogs in your
make.conf. You get all the information with:
PORTAGE_ELOG_CLASSES="warn error info log qa"
You can deactivate some of the information in case it is never helpful.
Also note that this does not save the whole build log. If a program fails to compile you will still have to emerge it again to see where the problem is.