According to upstream documentation https://github.com/ZoneMinder/zoneminder/blob/master/INSTALL zoneminder might be build without the ffmpeg. Ebuild has such a flag available and appropriate ON/OFF set in the configure phase, but DEPEND statement is missing appropriate flag which makes zoneminder pull to install ffmpeg anyways. ( virtual/ffmpeg ) should be changed into: ffmpeg? ( virtual/ffmpeg )
https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=c66a9574205fcefe13c0b13a778af774f2703a0b