Specify a llvm-config in Make.user when USE_SYSTEM_LLVM=1

There are more then one instances of LLVM installed on my box and the default version is 3.0, I want(must) use version 3.2 for compiling Julia, with this commit, it can be done easily by putting these lines in ``Make.user``:

```
USE_SYSTEM_LLVM = 1
LLVM_CONFIG = /usr/lib/llvm-3.2/bin/llvm-config
```