This bug affects systems with active rvm installations.
rrdtool formula choose whether to set --enable-site-dir configure flag based on comparison of which ruby to the system ruby path. Superenv succesfully strips all rvm pathes so formula supposes that I have the system ruby and tell rrdtool to install libraries to the system ruby sitedir instead of rvm's.
brew --config - https://gist.github.com/theirix/95239fca2284042406c0
brew doctor - https://gist.github.com/theirix/5dc5155386fd6498b6b2
Currently you need to use --env=std for PATH ordering to be preserved, there's really no way to have the best of both worlds in superenv.
rrdtool: use stdenv to support custom perl and ruby