Created attachment 511716 [details] emerge --info as in summary, sci-phisycs/root fails to build with error: 'RcppCommon.h' file not found the error happens if Rcpp is not installed in the system. looking at https://root.cern/building-root, the option r requires Rcpp and RInside form CRAN, but portage does not provide any ebuild to install them. see Bug #642346 for a patch.
Thank you for the report. Please *attach* the logfiles, https://wiki.gentoo.org/wiki/Attach_the_logs_to_the_bug_ticket and reopen this ticket (Status:unconfirmed).