Using the stock Python shipped by Apple with OS X 10.9 Mavericks and XCode 5.1, Mercurial (and other Python extensions) encounter an error like:

cc -fno-strict-aliasing -fno-common -dynamic -g -Os -pipe -fno-common -fno-strict-aliasing -fwrapv -mno-fused-madd -DENABLE_DTRACE -DMACOSX -DNDEBUG -Wall -Wstrict-prototypes -Wshorten-64-to-32 -DNDEBUG -g -fwrapv -Os -Wall -Wstrict-prototypes -DENABLE_DTRACE -pipe -I/System/Library/Frameworks/Python.framework/Versions/2.7/include/python2.7 -c mercurial/base85.c -o /private/var/folders/qv/tv81r5_9119bs3jwj3g2rp540000gn/T/hgtests._a6HZj/build/temp.macosx-10.9-intel-2.7/mercurial/base85.o
clang: error: unknown argument: '-mno-fused-madd' [-Wunused-command-line-argument-hard-error-in-future]
clang: note: this will be a hard error (cannot be downgraded to a warning) in the future
error: command 'cc' failed with exit status 1
make: *** [tests] Error 1
 
This can be worked around with:
 
export CFLAGS=-Qunused-arguments

On my machine, Python was build with clang-500.0.68 and the XCode 5.1 updates it to clang-503.0.40. I will include a simple setup.py that can be run against Mercurial's head to demonstrate the issue.