In effect that means extension builds that fail without llvm19-dev.tcz shouldn't be done at the moment, so perhaps it should be removed from mesa-dev.tcz.dep?
In my case it was pulled in by libwebp1-dev.tcz, but I don't think llvm19-dev.tcz will be needed for me to build the new version of Dillo with WebP support.
Anyway I have my own Mesa build without the LLVM dependency that I use, and before continuing I've been hacking the tce-install script (also tce-audit, and update-everything) to add support for the tcz-black.lst file that tce-update uses to block updating extensions matching the regular expressions there. At the moment tce-install keeps trying to replace my mesa.tcz (and mesa.tcz.dep, and replace or create mesa.tcz.md5.txt) when I try to download new extensions that depend on it like libwebp1-dev.tcz, and I've decided it's time for a permanent solution. Though at some point it's looking like I might have to really try and learn some AWK for that...