posted this on January 26, 2014, 22:11
(Revised March 14, 2014 - fix typo)
(April 28, 2014 - see Sturla tip in comments)
The latest version of XCode (version 5) that was released with OS X 10.9 Mavericks has removed support for gcc, such that gcc is no longer actually the GNU Compiler Collection, but is symlinked to the clang compiler.
Users still hoping to access a C compiler for their projects, such as in building C-extensions using Cython, should generally not run into any problems in using the symlinked gcc (or directly using clang), as clang uses the same LLVM backend and libraries as Apple's previous gcc compiler.
However, if you are building a C++ library (via clang++) that you will later link with a Python extension, or if you are building a Python extension that uses a C++ library, you need to use the older libraries (libstdc++, and not the clang++ default of libc++) via these compiler/linker flags: -stdlib=libstdc++ -mmacosx-version-min=10.6
As it may be preferable for some users, it is still possible to install and use Xcode 4.6.3 on an OSX 10.9, but note that these developer tools do not include the 10.9 SDK.