configure: added Gentoo to the supported distros.

--HG--
extra : amend_source : e11cf663d94b0fd4b4eb1f1fd47f0115aa52e533
add virtualenv to the gentoo install command.
configure: added missing dependency on virtualenv for Gentoo. Thanks to gubatron
Merge pull request #510 from ArneBab/master

configure: added Gentoo to the supported distros.