Lots of work, you need hundreds of steps across the snapshots, and patches for each one to get them to work. (E.g., the makefile had hardcoded -Werror for ages.) Not to mention that if you want to make it portable, you must always start with the i686 version and cross-compile from there. (Preferably leaving x86 as late as possible: the old LLVM versions are full of architecture-specific quirks.)