You can either use Xcode with gcc support, or, you can download the command line version of build tools for Mac.
If you want to use Xcode, Google for info on how to install gcc. E.g., http://www.cyberciti.biz/faq/howto-apple-mac-os-x-install-gcc-compiler/.
If you aren't already an Xcode user, I recommend you install only the command line tools.
- For an older Mac, get the command line tools from here: https://github.com/kennethreitz/osx-gcc-installer.
- For 10.7 and later, download from Apple's developer site, https://developer.apple.com/downloads/ (free account registration required); search for “Command Line Tools.”
Download and install from here: http://www.sfml-dev.org/download/sfml/2.1/