configure: added Gentoo to the supported distros.

This currently installs for the current user only, because otherwise it breaks existing python scripts.
gcc delivers g++ if the cxx useflag is set - which it is by default since 2009. libssl-dev should automatically be pulled with openssl: Gentoo every package includes the dev headers, because they are built locally and dependencies normally need the headers.

But virtualenv is missing - I added it. Thanks!