Compiling open source projects typically requires a C compiler and gcc is typically the compiler of choice. Mac OS X doesn't install the gcc compiler by default but it is freely available in the xcode suite of development tools.

To install the gcc compiler, download the xcode package from http://connect.apple.com/. You'll need to register for an Apple Developer Connection account. Once you've registered, login and click Download Software and then Developer Tools. Find the Download link next to Xcode Tools (version) - CD Image and click it!

Find the downloaded package, doubleclick it and follow the installation instructions to install gcc and a host of other development applications. gcc will be located at /usr/bin/gcc.

Websites tend to change over time, so if you notice that the steps to download xcode have changed, please post a comment and I'll update the recipe.