llvm_home needs to be set for the gradle build.
However, if it is not specified, we could also download it on demand, and store it in a known location (I'd suggest something other than the build directory, since then it will be deleted on `clean` as well).
However, if it is not specified, we could also download it on demand, and store it in a known location (I'd suggest something other than the build directory, since then it will be deleted on `clean` as well).