I have had a problem with compiling Pillow recently. the error was with Pillow/Pil compilation.
Apple updated command line tools. So the cc command was updated too.
The bug is in GCC. It is quite serious because those flags support will be dropped in future. Fortunately there is a worakround for now:
It works for Pillow 2.4.0 and I can compile it with Apple LLVM 5.1 (That is current now), as of Xcode 5.1.
Hope my findings will help someone.