Modified the Makefile to support a different Homebrew root folder.

While working on an Erlang project, I wanted to use Lazybuilder to compile the project automatically; I'm lazy =)

I discovered that the Makefile used in this project requires the default location of Brew. Since I'm using Boxen to manage my laptop, this caused an issue since the default location is /opt/boxen/homebrew/bin/brew for boxen. 

Attach is a change that supports using the HOMEBREW_ROOT environment variable when present, otherwise it'll default to /usr/local.