Currently, evolution-data-server has www-client/mozilla in its DEPEND variable. This forces the installation of www-client/mozilla even though the user might already have www-client/mozilla-bin installed on their system. I think the dependency should be changed to: mozilla? ( || ( www-client/mozilla www-client/mozilla-bin ) )
It depends on the source version since it requires the relevant header files and libraries for compilation. People with mozilla-bin installed should emerge evolution-data-server with USE="-mozilla" so only the individual nps/nss libraries are pulled as dependencies.
