For as long as I can remember #build has been the Release Engineering (née Build & Release) home on IRC. We've learned over the years that most people who stop by #build randomly ask questions related to the build system, not the infrastructure. This ends up with us redirecting them to #pymake, #developers, or other channels where they're more likely to get help. In order to help avoid this confusion we've decided to move RelEng to #releng and let #build be the home for the build system, likely to replace #pymake.
If you've got build infrastructure questions or just like to hang out with us, join us in #releng!