Build continually runs make on LLVM

After doing work on LLVM, the build dependencies get confused and everytime Rust's make is run, it also runs make on LLVM, even when there's nothing to do.

This is probably because our rules for building LLVM are not very exact - we use llvm-config as our target, which is just a script so probably doesn't always get updated when building LLVM. Running touch on llvm-config would probably fix this.
