berkdb - Enable sys-libs/db database backend for header caching
debug - Enable extra debug codepaths, like asserts and extra output. If you want to get meaningful backtraces see https://wiki.gentoo.org/wiki/Project:Quality_Assurance/Backtraces
doc - Add extra documentation (API, Javadoc, etc). It is recommended to enable per package instead of globally
gdbm - Enable sys-libs/gdbm database backend for header caching
gnutls - Prefer net-libs/gnutls as SSL/TLS provider (ineffective with USE=-ssl)
gpgme - Build gpgme backend to support S/MIME, PGP/MIME and traditional/inline PGP
gsasl - Use GNU SASL via virtual/gsasl instead of Cyrus SASL (requires USE=sasl)
hcache - Enable header cache, one database backend needs to be enabled
idn - Enable support for Internationalized Domain Names
imap - Add support for IMAP (Internet Mail Application Protocol)
ssl - Add support for SSL/TLS connections (Secure Socket Layer / Transport Layer Security)
tokyocabinet - Enable dev-db/tokyocabinet database backend for header caching
vanilla - Do not add extra patches which change default behaviour; DO NOT USE THIS ON A GLOBAL SCALE as the severity of the meaning changes drastically