Turns out the wholesale switch to clang/llvm is causing more trouble with other python packages along with M2Crypto.

Having a problem installing the excellent pandas python library on Mac OS X 10.9 Mavericks?

You may be seeing an error like this when running sudo pip install pandas

clang: error: unknown argument: '-mno-fused-madd' [-Wunused-command-line-argument-hard-error-in-future]

Thankfully, there is a simple solution.  As root:

export ARCHFLAGS="-Wno-error=unused-command-line-argument-hard-error-in-future"

pip install pandas