Latest fglrx drivers has a new compile flag: --NoXServer
From the fglrx manual:
GPUs can be used only for computation purposes (commonly seen with AMD server graphics products or headless GPUs) where X Server is running on non-AMD GPUs or in cases where there is no X Server running on the system.
AMD provides support for headless implementations of server GPUs.
It would be nice if a new USE can install the fglrx driver without the xorg dependency.
The flag --NoXServer seem to work from 15.7 version.
x11-drivers/ati-drivers has been removed, per bug 582406.