xinerama seems to be an optional dependency for xdm but it was forced in our ebuilds. i've written a patch which could be used for xdm-1.1.6.ebuild and xdm-1.1.8.ebuild (other ebuilds not tested).
Created attachment 176513 [details] xdm-1.1.6.ebuild.patch
The patch isn't good enough as it will introduce an automagic dependency. There the xinerama USE flag needs to induce a modification of the configure options to remove all risks. Thanks
The configure.ac needs a change so that it takes a --with-xinerama flag. This is an upstream change, so please file a bug at bugs.freedesktop.org, with a patch if possible. If you would like, feel free to post a link to it here. Thanks!