Do NOT install osx-gcc-installer as @gcamp suggested.
I did, and it corrupted my system files to an extent that I had to reinstall OSX.
This is how you uninstall osx-gcc-installer completely, and restore your system files:
http://magnemg.tumblr.com/post/113251602430/how-to-completely-uninstall-osx-gcc-installer
You probably need to install Xcode (I did, when having to upgrade to Qt 5, which had a hard dependency on Xcode. There was no way around it.). Or you might be lucky and can get away with just installing the official Apple CLT:
Apple now provides an official Command Line Tools for Xcode package
that you can install without needing to install Xcode itself! You can
download it from Apple's developer site (free registration required)
and search for "Command Line Tools".
See also:
https://stackoverflow.com/a/20671590/380607 which says:
It appears that OSX has changed xcodebuild to require XCode to be
installed, where before it functioned properly with only the OSX
Command Line Tools installed.