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
gnutls - Prefer net-libs/gnutls as SSL/TLS provider (ineffective with USE=-ssl)
iconv - Enable support for the iconv character set conversion library
icu - Enable ICU (Internationalization Components for Unicode) support, using dev-libs/icu
libffi - Enable support for Foreign Function Interface library
zeroconf - Support for DNS Service Discovery (DNS-SD)
