Gentoo started using the APACHE2_MODULES USE-flag-extending variable to manage Apache's modules in portage some time ago. The list of modules I migrated with (likely provided by the Gentoo documentation as a recommendation) seems to be missing those responsible for http authentication so every now and then on an outdated web server I will run into errors when adding directives like AuthUserFile.
To resolve this, make sure the following modules are contained in the APACHE2_MODULES variable in your make.conf:
authn_file auth_basic auth_digest authz_user
Recompile Apache with the --newuse flag:
# emerge --update --newuse --deep apache --ask