The LLVM compiler in Xcode 5.1 now treats unrecognized compiler options as errors by default. Building Python or Ruby modules using unrecognized flags will now cause the build to fail.
I ran into this when trying to build lxml for Python recently. This failed with the following error message:
clang: error: unknown argument: '-mno-fused-madd'
Google or stackoverflow provided no answer to this problem.
As long as the build configurations are not updated an up-to-date OS X system will currently fail to build modules like lxml for Python.
Luckily a (temporary) solution is available in the the Xcode release notes:
The Apple LLVM compiler in Xcode 5.1 treats unrecognized command-line options as errors.
To workaround this issue, set the ARCHFLAGS environment variable to downgrade the error to a warning.
Use the following to successfully build lxml on Mavericks with LLVM/Xcode 5.1:
sudo ARCHFLAGS=-Wno-error=unused-command-line-argument-hard-error-in-future pip install lxml