-------
Your Welcome.
Easy way to find the def of that USE flag ( it gave away the error ) :
~ # equery u media-libs/mesa
+ + opencl : Enable the Clover Gallium OpenCL state tracker.
Easy way to check what else currently installed that may be using that
USE flag :
~ # equery h opencl
* Searching for USE flag opencl ...
[IP-] [ ] media-gfx/imagemagick-6.9.7.4:0/
6.9.7.4
[IP-] [ ] media-libs/mesa-17.0.0_rc2:0
[IP-] [ ] media-libs/x264-0.0.20160712:0/148
[IP-] [ ] sys-apps/hwloc-1.11.2:0/5
Corbin