Add PKG_CXXFLAGS to Makevars

Setting the PKG_CXXFLAGS flag to -std=c++11 allows Rcpp to correctly use the c++11 standard in Linux Mint. I'm not really sure why the c++11 flag that's there already isn't recognized, but this fixes the problem. I see absolutely no reason why this change should cause any problem on any other system, but maybe someone should at least build with it on a Mac before this gets merged in.