On a prefix install with a sufficiently old host glibc, every single package installation logs the message "Your host glibc is too old; enabling -fgnu89-inline compiler flag. bug 473524" (from profiles/prefix/linux/profile.bashrc). This completely swamps portage's log summary output after an operation on any significant number of packages, making it hard to pick out the messages that might actually require some action. Please downgrade the message to an einfo.
Fixed, thanks!