It seems that the latest unmasked ati-drivers, ati-drivers-15.9-r1, cannot build against recent kernels, e.g. 4.4.6.
I notice that ati-drivers-15.12-r1 is intended to address this problem but it still doesn't build. So I'm reporting it here.
* IMPORTANT: 31 news items need reading for repository 'gentoo'.
* Last emerge --sync was 1y 6d 4h 40m 24s ago.
* Evergreen chipset and newer.
* This includes the AMD Radeon HD 5400+ series at this moment.
I looked briefly at porting forward the unmasked ati-drivers ebuild but quickly got out of my depth in kernel changes between my 3.8.13 kernel and 4.4.6.