You receive this bug because this package does not respect my system's CXX (x86_64-pc-linux-gnu-g++) and calls directly /usr/bin/g++(or similar) The possible solutions to fix this issue are: 1)Fix the buildsystem, if you can; 2)inherit toolchain-funcs and use tc-export CXX 3)inherit toolchain-funcs and use emake CXX="$(tc-getCXX)" From the build log https://476684.bugs.gentoo.org/attachment.cgi?id=353182 : g++ -I. -I. -I/usr/lib64/ruby/1.8/x86_64-linux -I. -DHAVE_MECAB_H -fPIC -march=native -O2 -g0 -pipe -fno-strict-aliasing -fPIC -c MeCab_wrap.cpp
mkmf.rb on ruby18 lacks CXX support. I've added tc-export CXX to ruby18 capable ebuilds.