Gentoo's Bugzilla –
Attachment 396954 Details for
Bug 540664x11-drivers/xf86-video-mga-1.6.3 USE=dri with x11-base/xorg-server-1.17.* - .../work/xf86-video-mga-1.6.3/src/mga_dri.c:1168:18: error: 'DRIWrappedFuncsRec' has no member named 'ValidateTree'