GLAMOR provides 2D acceleration on the Radeon HD 7000 "Southern Islands" GPUs and newer Radeon GPUs via OpenGL rather than implementing the 2D hardware acceleration by hand with EXA in the DDX driver. While HD 7000 series hardware is now common place and the RadeonSI driver maturing well in recent months, GLAMOR is now being enabled by default at build time for the xf86-video-ati driver.
Building the Radeon driver with GLAMOR support still requires the independent GLAMOR library and header files so if your system is lacking them you need to configure the xf86-video-ati driver now with the --disable-glamor build switch. GLAMOR is only mandatory to use for 2D acceleration on the HD 7000 series ASICs and newer but it can also be optionally used by older Radeon GPUs by setting the AccelMethod value in the xorg.conf to glamor and making the appropriate glamoregl change to the modules section of the X.Org configuration.
The change to attempt to build GLAMOR by default in the xf86-video-ati driver happened with this Git commit this morning.