Just a note that Python-3.5.0rc1 fails to compile on Mac OS X 10.8.5 with the following compiler:

bash$ clang --version
Apple LLVM version 4.2 (clang-425.0.28) (based on LLVM 3.2svn)
Target: x86_64-apple-darwin12.6.0
Thread model: posix
bash$ 

Here is the resulting compilation error:

/usr/bin/clang -c -Wno-unused-result -Wsign-compare -Wunreachable-code -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes    -Werror=declaration-after-statement   -I. -IInclude -I./Include    -DPy_BUILD_CORE -o Python/ceval.o Python/ceval.c
fatal error: error in backend: Cannot select: 0x102725710: i8,ch =
      AtomicSwap 0x102c45ce0, 0x102725010, 0x102725510<Volatile
      ST1[@gil_drop_request.0.b]> [ID=7]
  0x102725010: i64 = X86ISD::WrapperRIP 0x102723710 [ID=6]
    0x102723710: i64 = TargetGlobalAddress<i1* @gil_drop_request.0.b> 0 [ID=4]
  0x102725510: i8 = Constant<1> [ID=2]
In function: take_gil
make: *** [Python/ceval.o] Error 1

Problem can be fixed by commenting out the following line in pyconfig.h

/* Has builtin atomics */
// #define HAVE_BUILTIN_ATOMIC 1   


Not really sure what to advise.  To my eyes, it looks like a bug in clang or Xcode.  So, maybe this is more just an FYI that source builds might fail on certain older Mac systems.