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.
Good, I'll do your suggestions.