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