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?
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.