Apple has decided not to ship the gcc compiler (used, among other things, to compile C programs for ECS30) in their latest OS X release named Mountain Lion.
To install gcc, please follow read the following FAQ on Apple's developer website for installing XCode, specifically the sections titled How do I download Xcode, and the OS X and iOS SDKs? and I've installed Xcode 4.3 or later. Where can I find the updated command line tools?.
You must install the command line tools in addition to XCode for access to gcc.
Note that this does not apply to earlier releases of OS X preceeding Mountain Lion.