Search for llvm-config on OSX using brew

On Mac OSX, this command fails: `ruby-install rbx`.

The reason is that the OSX LLVM does not show up as expected. 

Many OSX developers install an additional LLVM by using the `brew` package manager: `brew install llvm`.

The `brew` manager does not replace the existing system LLVM because that would cause problems with other OSX software, for example XCode. Any approach that uses `brew link` is potentially dangerous on OSX.

Therefore, this patch adds a search for llvm-config: if the system is Darwin, and the system says that `brew` is installed, and `brew` says the formula for LLVM is installed, then the code searches for the `llvm-config` file.

I expect this code will solve the `ruby-install rbx` issue for the majority of OSX brew users. 

If this code is accepted, then subsequent improvements could add logging, advice, and corner cases such as multiple brew LLVM installations.
@joelparkerhenderson lol, you did the polar opposite of what I suggested, I meant:

```ruby
elsif @darwin
  out = `brew list llvm | grep '/llvm-config$'`  # regardless of whether `brew` or the llvm formula are present
  config = out.chomp if $?.success?
else
  # ...
```