Michel Dänzer authored Sep 14, 2016 and Michel Dänzer committed Sep 14, 2016

We were using the result of `pkg-config --variable=sysconfigdir
xorg-server` before, which may not be inside $prefix, so make install
might fail for 10-amdgpu.conf .

Fixes make distcheck in that case, and possibly also 10-amdgpu.conf
seemingly missing from some distribution packages.

This matches what some (though not all) input drivers are doing for their
xorg.conf.d snippets.