Search for and use a versioned llvm-config if available

This enables automatic buildtime compatibility with [the Debian and Ubuntu packages hosted at llvm.org](http://llvm.org/apt/). Priority is given to the versioned llvm-config (e.g. llvm-config-3.3) to better support systems with multiple LLVMs installed.

This should ease the build on Travis CI machines, which run Ubuntu 12.04.