dad

I got that issue today too. Was this other package jsoncpp?
 
I got around it by emerging an older version of cmake, cmake-2.8.12.2, then emerging jsoncpp, and updating cmake to current.
 
To have portage emerge the older version, try echo ">dev-util/cmake-2.8.12.2" >> /etc/portage/package.mask
Once that's done, I removed the line from package.mask
 
Hope it works for you!